Jenkins et al.—Four new species of seep-restricted bivalve Caspiconcha
scar, but differs by having a narrow hinge plate and a more pointed anterior shell; C. whithami has a broad hinge plate and a broad anterior shell. The more pointed anterior side also dis- tinguishes Caspiconcha basquensis from the similarly straight and elongate species C. rubani. Caspiconcha basquensis differs from all other known species of Caspiconcha by having a shallower anterior adductor muscle scar.
Caspiconcha yubariensis new species Figure 4
2013 Caspiconcha sp. Jenkins et al., p. 371, fig. 11. Holotype.—Best preserved specimen (NMNS PM27843). Paratypes.—NMNS PM25523-25525, 27837-27842.
Diagnosis.—Shell modioliform to subtrapezoidal; strong ridge from the umbo to posteroventral margin, relatively narrow but clear depression runs below the ridge; varied angulation in anteroventral area especially in adult specimen; deep-set ante- rior muscle scar; strong internal ridge.
Occurrence.—Utagoesawa seep site in late Albian (Early Cre- taceous) deposit of ‘main part formation,’ Yezo Group exposed along the Utagoesawa Creek, Yubari City, Hokkaido, Japan.
Description.—The shell large, up to 247mm (NMNS PM25523); modioliform to subtrapezoidal, equivalve, strongly
inequilateral, strong inflation in the dorsal part of the central flank running obliquely toward posterior part of the ventral margin, relatively well-developed triangular depression runs from the umbonal area widening toward the mid flank; dorsal margin slightly curved along its whole extent, posterior margin rounded, angulation of shell shape in anteroventral part, strength of angulation varies among species; umbones close to the anterior end, situated at, or slightly below, the hinge line; shell thick, especially in the anterior part; exterior ornament consists of commarginal growth lines, more pronounced on the shell anterior; shell interior generally smooth, large elongated ante- rior adductor muscle scar bordered posteriorly by a stout myo- phoric buttress, strong oblique ridge running from umbonal area toward posterodorsal area, probable posterior adductor muscle scar situated just above the oblique ridge in posterodorsal area; hinge stout with very long ligament groove.
Etymology.—For Yubari City, from the type area of the species.
Remarks.—Angulation of the shell shape in anteroventral part varies among observed specimens; for example, adult speci- mens of NMNS PM25523 (Fig. 4), 27837 and 27839 have strong angulation, although juvenile (paratype NMNS PM27841, Fig. 4) and young (holotype NMNS PM27843, Fig. 4) specimens have very weak angulation. Caspiconcha yubariensis n. sp. is similar to the type species C. whithami in general shell shape, but differs by having a much stronger ridge and a relatively narrow and strong depression from the umbonal area toward the posteroventral area. Caspiconcha yubariensis is also similar to C. raukumaraensis n. sp. in general shell shape, but
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C. raukumaraensis differs by its shallow myophoric buttress and the lack of internal ridge. Caspiconcha yubariensis differs from other known Caspiconcha species by having angular anteroventral margins.
Caspiconcha raukumaraensis new species Figures 5, 6
2013 Caspiconcha sp. Kiel et al., p. 26, fig. 13D, E. Holotype.—TM8832.
Paratypes.—TM8831 (anterior portion of right valve; L= 56.4mm), TM9255 (anterior portion of left valve; L=36mm), TM92556-9 (total: 14 specimens from GS668, one from Awanui II)
Diagnosis.—Shells strongly recurved as adults, with broad tri- angular depression in mid flank; anterior adductor muscle scar subrectangular, anterodorsal side deeply impressed, and pos- teroventral side projected.
Occurrence.—Seep carbonates in undifferentiated Whangai– Wanstead formations, mid-Cenomanian at Port Awanui, North Island, New Zealand (FRF locality number Z15/f7485).
Description.—Shell recurved modioliform, up to 155mm long; umbo subterminal, prosogyrate, very low; dorsal margin slightly to moderately curved with thick, external ligament; posterior margin broad, convex; ventral margin slightly concave; anterior margin short and blunt; shell inflated near umbo with curved, blunt ridge running toward the posteroventral margin, broad triangular depression below ridge; surface with rough, irregular growth increments. Shell interior with numerous small pits above pallial line in
posterior third of shell (mantle muscle scar area); faint, broad band running from umbo toward posterior end of shell; anterior adductor muscle scar subrectangular, slightly longer than high, ventral margin moderately convex, covered by commarginal growth increments, anterodorsal side deeply impressed; pedal retractor scar triangular, situated posterodorsal of adductor muscle scar and separated from it only by weak
ridge.Myophoric buttress steep; caspiconchiid process present; hinge edentulous. Pallial line entire, indistinct, starting at posterior side of anterior adductor muscle scar, curved relative to ventral margin, poster- iorly distant from shellmargin, ending at posteroventral corner of posterior adductor muscle scar. Posterior adductor muscle scar weak, only known from a single small (42mm long) specimen, elongate oval, parallel to posterodorsal shell margin, indistinct dorsally, ventrally with fine growth increments.
Etymology.—For the Raukumara Peninsula in northeastern New Zealand.
Remarks.—Caspiconcha raukumaraensis n. sp. is variable in shell shape, with nearly straight-sided specimens to recurved specimens. This variation may to some extent represent an ontogenetic change because all available small specimens are nearly straight-sided, while most, but not all, large specimens show curvature. At least in one specimen, the extreme concavity of the ventral margin appears to be due to an injury in this part of
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