16 * MAZATLAK BIVALVES 



terior part. Epidennis not conspicuous. Lunular portion deeph" 

 impressed : umbos distant from margin. Hinge without spatli- 

 idate lamina : mantle marks rugose. More or less elongated, 

 api)roaching sometimes the more ovate forms of truucata, especi- 

 ally in the yoimg shells, but distinguished at once by the hinge. 

 The anterior margin in the young shells is generally serrated, 

 like Pholas, more strongly than in truncata. Anterior por- 

 tion either more or less than one-fifth of the entire length. 

 The largest specimen measures loi7g. "76, lat. I'l, alt. 6 "2. 

 Smallest, lonf/. '04, lat. '08. 



Sah. — Isle of Perico (Bay of Panama) and Isle of Plata, Cum- 

 ing.— 'M.SLZB.tla.n ; in Spond3'lus Lamarekii, very rare ; L'pool 

 Jj* Havre Coll. 



Tablet 56, 1 valve and 2 pairs very young. — 57, young speci- 

 men in situ. — 58, 2 adult specimens : one hlac, fresh ; the other 

 white, large, dead. — 59, fragments of shelly lining. 



Family SAXICAYID^. 

 Genus SAXICAVA, Bellenue. 



23. Saxicava aectica, Lin. 



(For the synonyms, both as to genus and species, of this 

 Protean shell, see B. M. Cat. Brit. An. Part VII. pp. 86—89.) 



? = Saxicava sohda. Sow. Proc. Zool. Soc. 1834, p. 88: D'Orlj. 

 Vo/y. V. 521. 



? = Saxicava Cordieri, Gould, ms. — Mazatlan, Lieut. Green. 



This shell is named on the authority of E. M'Andrew, Esq. 

 whose practical acquaintance with the fomis it assumes both in 

 Arctic and subti'opical regions renders his opinion of weight. 

 According to Hanlej-, Menke and Gould, it is a distinct species. 

 I cannot separate some forms of it from solida. The creature 

 seems to take delight in distorting itself, not thinking it neces- 

 sary to lodge in an uneven cavity for that purpose. It seldom 

 (if ever) bores, being foimd in cavities of Lithodomus &c. The 

 young shells are just as variable as the old ones, somettmea 

 assuming the characters of S. rugosa or the form of Spha>nia 

 Binghami, but generally displaying the narrow, anteriorly trun- 

 cate, bi-tuberculate character of S. arctica. It rarely takes the 

 genuine Hiatella form. Ligament of variable length, generally 

 rather short. Valves capable of opening widely, though rarely 



