572 DR. GWYN JEFFREYS ON THE MOLLUSCA OF THE [June 17, 



Say's description of A. transversa and the figures lately given by 

 Biimey and Tryon, as well as from specimens which Mr. Dall has 

 kindly sent me, I am inclined to consider it either the same as the 

 present species or at most a variety of it. In Weinkauff's collec- 

 tion of Algerian shells is a specimen three and a half inches long, 

 with 32 ribs, the usual number being 28. A monstrosity in the 

 same collection was named by M. Crosse A. weinkavffi. There are 

 several other synonyms. 



7. Arca oblio.ua, Philippi. 

 t/ A. obliqua, Phil. Moll. Sic. ii. p. 43, t. xv. f . 2 : B. C. ii. p. 175 ; 

 v. p. 175, pi. xxx. f. 4. 



' Lightning ' Exp. : St. 5. 



< Porcupine' Exp. 1869 : St. 14, 15, 25, 65. 1870 : Ail. 2, 3, 24, 

 25, C. Sagres, 26-30, 36 ; Med. 45, 55, 58, Adventure Bank, off 

 Rinaldo's Chair. 



Distribution. Bergen and Shetland to the iEgean, Azores ; 30- 

 600 fms. 



Fossil. Pliocene. S. France, Calabria, and Sicily. 



Having now reexamined a great number of recent and fossil spe- 

 cimens from various localities, I must separate this from the next 

 species for the following reasons : — A. obliqua is not merely much 

 smaller, but it is shorter (measured from the beak to the front 

 margin), and more sharply angulated on the anterior side ; the strife 

 are nodulous ; and the teeth are more numerous, and straight 

 instead of being set obliquely on the anterior side, as in A. glacialis. 

 Both species are somewhat inequivalve. Some specimens of A. ob- 

 liqua have the inside of the front margin regularly and closely den- 

 ticulated. 



l/ 8. Arca glacialis, Gray. 



A. glacialis, Gray in Suppl. App. Parry's first voyage, p. ccxliv ; 

 Torell, Spitzb. Moll t. ii. f. 7, a, b. 



' Porcupine' Exp. 1869 : St. 23 a, 89. Valves only, and perhaps 

 semifossil relics of the last glacial epoch. 



Distribution. Arctic seas in both northern hemispheres, Iceland, 

 and G. of St. Lawrence ; 25-1622 fms. 



Fossil. Post-tertiary. Scandinavia, Maine ; 0-240 ft. 



Not A. glacialis of Mighels. 



9. Arca pectunculoides, Scacchi. 



A. pectunculoides, Sc. Ann. Civ. due Sic. vii. p. 82 (1833) : B. C. 

 ii. p. 171 ; v. p. 175, pi. xxx. f. 3. 



' Lightning ' Exp. : St. 2, 5, 7. 



' Porcupine' Exp. 1869 : St. 1, 6, 14, 15, 17, 23, 25, 61, 62, 65, 

 off Lerwick. 1870: Atl. 1, 2, 3, 6, 8, 9, 12, 13, 14, Setubal B., 24 

 -34 ; Med. 45, Cartagena B., 55, Benzert Road, Adventure Bank, 

 off Rinaldo's Chair. 



Distribution. Davis Strait to G. of St. Lawrence and Halifax. 



