6 MAZATLAN BIVALVES 
terior part. Epidermis not conspicuous. Lunular portion deeply 
impressed : umbos distant from margin. Hinge without spath- 
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 Jong. °76, lat. 1°1, alt. 6°2. 
Smallest, Jong. *04, lat. 08. 
Hab.—\sle of Perico (Bay of Panama) and Isle of Plata, Cwm- 
ing.—Mazatlan ; inSpondylus Lamarcki, very rare; L’poo/ 
& 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. 
Famity SAXICAVIDA. 
Genus SAXICAVA, Bellevue. 
23. SAXICAVA arctica, Lin. 
(For the synonyms, both as to genus and species, of this 
Protean shell, see B. M. Cat. Brit. An. Part VIL. pp. 86—89.) 
?=Saxicava solida, Sow. Proc. Zool. Soc. 1834, p.88: .D’ Orb. 
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 Lithodomus &c. The 
young shells are just as variable as the old ones, sometimes 
assuming the characters of S. rugosa or the form of Sphenia 
Binghami, but generally displaying the narrow, anteriorly trun- 
cate, bi-tuberculate character of 8. arctica. It rarely takes the 
genuine Hiatella form. Ligament of variable length, generally 
rather short. Valves capable of opening widely, though rarely 
