156 



ARCADE. 



Fig. 463. 



at the point, even more so than the anterior end ; the valves are 

 tumid, and only a very narrow portion of the posterior dorsal edge is 

 compressed ; apices obtuse ; ventral margin gently and 

 uniformly arched ; surface very delicately striated con- 

 centrically, and covered with a pale ■ yellowish-green 

 epidermis ; hinge very delicate, with ten teeth in front 

 and twelve behind the apex. Length, three twelfths of 

 an inch ; height, one eighth of an inch ; breadth, one 

 tenth of an inch. 

 Taken in deep water in Massachusetts Bay. 



It is narrower and more inflated than the young of Y. thracice- 

 formis, and is quite remarkable for its regularly oval, non-rostrate 

 form. It is very similar in size and general appearance to Yoldia 

 pijgmcca^ Miinst, but that shell is pretty distinctly pointed and 

 slightly upturned at the end. 



Y. obesa. 



2 

 T 



Yoldia siliqua. 



Shell oblong-ovate, emarginate under the posterior tip ; beaks nearly median, 

 prominent; posterior dorsal margin straight, compressed. 



Nucula (jlaciaUs, Gray, in Index Test. Supp. pi. 2, fig. 6. — Hisixger, Icon. Pctr. Suec. 

 pi. 30, fig. 13 (fossil) ; LetlKL-a Suec. 60. 



Nucula iruncala. Brown, Conch. Gr. Brit. 84, pi. 33, fig. 18. 



Nucula Portlandica, Hitchcock, Bost. Journ. i. 327 (woodcut 328). — Reeve, in Belch- 

 er's last Arct. Voy. 396, pi. 33, fig. 3. 



Nucula siliqua, Reeve, in Belcher's last Arct. Voy. pi. 33, fig. 4 (185.5) ; Proc. Zool. Soc. 

 48 (185G). 



Nuculana f/larialis, Morch, Prodr. Moll. Groenl. 21 (1857). 



Leda (suhg. Portlundia) glacialis, IIanley, in Thes. Conch, iv. 144, pi. 227, figs. 31, 32 

 (1860). 



Yoldia glacialis, Gray, Catal. Br. Moll. 161 (1851). 



Shell oblong-oval, ventricose, beaks large and unusually promi- 

 nent, a little in advance of the middle ; anterior end rounded, rather 

 acutely ; ventral margin very gently curved, a little 

 pouched opposite to the beaks and also at the ter- 

 mination of the umbonal ridge, between which and 

 the posterior point the ascending margin is a little 

 concave ; the posterior dorsal edge is sharp and 

 straight, and meeting the ascending margin forms an 

 acute-angled point ; this caudal portion is very much 

 Y. siliqua compressed ; the dorsal area is flattened and de- 



fined by an obtuse ridge ; surface somewhat coarsely 

 waved, and marked also by delicate incremental undulating ridges of 



Fitr. 464. 



