156 ARCADE. 



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 

 ' 2 ' 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 

 pyf^mcea^ Mlinst, but that shell is pretty distinctly pointed and 

 slightly upturned at the end. 



Yoldia siliqua. 



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

 prominent; posterior dorsal margin straight, compressed. 



Nucula glacialis. Gray, in Index Test. Supp. pi. 2, fig. 6. — Hisinger, Icon. Petr. Suec. 

 pi. 30, fig. 13 (fossil) ; Lethiea Suec. 60. 



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



Nucula Portlandica, Hitchcock, Best. 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 (1855) ; Proc. Zool. Soc. 

 48 (1856). 



Nucularta (/lacialis, MoRCH, Prodr. Moll. Groenl. 21 (1857). 



Lecla (suhg. Portlandia) glacialis, Hanley, 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, ratlier 

 acutely ; ventral margin very gently curved, a little 

 '^'^" ^' pouched opposite to the beaks and also at the ter- 



mination of the umbonal rido-e, 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 

 i.sutqua. 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 



