24 MAZATLAN BIVALVES 



GENUS SPH^^TA, Turton. 



For Monograph, of this genus, with amended generic characters, 

 see A. Adams in Proc. Zool. Soc. 1850, p. 86. See also Forbes 

 fy Hani. Br. Mol. vol. 1. p. 189193 -.Clark Br. Mar. Test. 

 Moll. p. 150. 



35. SPHJENIA FRAGILIS, n. s. 



S. animali in cryptis latibulante, ergo varie distorto ; testa 

 parvd, tenui, subnacred, vix rugose striatd ; epidermide fusco- 

 virente copiose indutd, rug arum increscentium concentricarum 

 plena, postice in siphone longd porrectd : parle posticd plus 

 minusve subcarinatd ; valvd sinistrd dente lig amentum ferente, 

 plus minusve seuprolongatd seu extante; dextrd alveo conveniente, 

 nonnunquam denticulo subextante : impressionibus muscularibus 

 subrotundatis, sinu pallii lato, rotundato, hand alto. 



It is surprising how much of the very minute description of 

 S. Binghami given in Forbes fy Hani. Br. Moll. i. 191-2, applies 

 exactly to individuals of this species. Indeed, if young speci- 

 mens of the two were mixed together, I should hardly know 

 any sufficient ground of specific distinction, except in the texture 

 which is more nacreous, and the pallia! sinus which is broad, 

 though shallow. The young shells can sometimes be told from 

 those of Saxicava arctica only by the hinge, as in Binghami ; 

 and there is often seen the little denticle by the ligament pit 

 noticed by Turton, not Hanley, and conspicuous in young 

 shells of Sphsenia Binghami in my possession, nestling in cre- 

 vices of limestone dredged off Weymouth. Like other nestlers 

 (unlike the true borers, which are moderately constant in form) 

 it is extremely irregular. Many well characterized species 

 might be made out of extreme forms ; but unfortunately 

 for tne lovers of multiplication, individuals were sufficiently 

 numerous to supply many connecting links. The normal state 

 appears to be not very inequilateral and tolerably well rounded : 

 the shell is then shaped somewhat like Psammobia : but it is 

 generally more or less produced, when the posterior portion 

 becomes marked off by an angle, in very long specimens 

 amounting to a keel, sometimes with a trace of a double one. 

 When it lives in dead Balani &c., it becomes very short, inflated 

 and gibbous, resembling Corbula o"r sometimes JN"esera. The 

 ligamental plate then becomes narrow, projecting and sinuated, 

 more like the tooth of Mya. These variations are seen in the 



