16 MAZATLAN BIVALVES 



terior part. Epidermis not conspicuous. Lunular portion deeply 

 impressed : umbos distant from mprgin. Hinge without spatli- 

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

 approaching sometimes the more ovate forms of truncata, especi- 

 ally in the young 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 long. '76, lat. 1*1, alt. 6 '2. 

 Smallest, long. '04, lat. '08. 

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



ing. Mazatlan ; in Spondjdus Lamarckii, very rare ; L'pool 



^' Havre Coll. 



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

 men in situ. 58, 2 adult specimens ; one lilac, fresh ; the other 

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



FAMILY SAXICAVID.E. 

 GENUS SAXICAVA, Sellevue. 



f 23. SAXICAVA AECTICA, Lin. 



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

 Protean sheU, see B. M. Cat. frit. An. Part VII. pp. 8689.) 



? = Saxicava solida, Soiv. Proc. Zool. Soc. 1834, p. 88: D'Orl. 

 Voy. v. 521. 



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



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

 whose practical acquaintance with the forms it assumes both in 

 Arctic and subtropical regions renders his opinion of weight. 

 According to Hanley, 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 found in cavities of Lithodornus &c. The 

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

 assuming the characters of S. rugosa or the form of Sphsenia 

 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 



