156 



arcadj:. 



Fig. 4G3. 



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 

 unitbrnily arched ; surface very delicately striated con- 

 centrically, and covered with a pale yellowish-green 

 epidermis ; hinge very delicate, witli ten teeth in front 

 and twelve behind the apex. Length, three twclftlis of 

 an inch ; heiglit, 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. thracia- 

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

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

 pj/iy-m(ro, Miinst, but that shell is pretty distinctly pointed and 

 slightly upturned at the end. 



Yoldia siliqua. 



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

 prominent; posterior dorsal margin straight, compressed. 



Nucuki (jlacialis, Gray, in Index Test. Supp. pi. 2, fig. 6. — IIisinger, Icon. Petr. Suec. 

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



Nucttla tninca/a, Brown, Conch. Gr. Brit. 84, pi. 33, fig. 18. 



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

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



Nucula siliqua, Reeve, in Belcher's hist Arct. Voy. pi. 33, fig. 4 (185.i) ; Proc. Zool. Soc. 

 48 (1856). 



Nuculana qkic.ialis, Morch, Prodr. Moll. Groenl. 21 (1857). 



L&la [sulu]. Portlandia) jlacialis, H.\nley, in Thes. Conch, iv. 144, pi. 227, figs. 31, 32 

 (18G0). 



Yoldia iilacialis, 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 tlie ter- 

 mination of the uml)onal ridge, l)etwecn 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 

 ^ ,. comiiressed ; the dorsal area is flattened and de- 



Y. siliqua. " 1 ' 



fined by an ol)tuse ridge ; surface somewhat coarsely 

 waved, and marked also by delicate incremental undiflating ridges of 



Tk. 464. 



