MAZATIAN BIVALVES 125 



ventrali incurvo ; epidermide fusca, rugis concentricis parte 

 posteriore instnictd ; incrustatione in ^jwr/e posticd solidd, in 

 lineas duas ah umhonihus decur)'ente,st)'iis sagittiformihusincon- 

 spicuis ad marginem directis ; in rostra solida prolongatd, 

 appress a , maxima parte non excavata sed ad 

 ap icem aJtc effbssa, qtias i calij ce m adhiheniia. 



The only specimen found is in shape like L. arist. tumidior, 

 but differs in the remarkable character of the incrustation. 

 This lies in a solid triangular laj^er over the posterior part, 

 with arrow-headed lines pointing away from the vertex of the 

 triangle. At the sides, the coarse rugfc of the epidermis are 

 visible, ending in a diagonal line bounding the posterior part. 

 The incrusting beaks are appressed, as in L. attemiatus, but 

 are not hollowed within, as in that species, until the extremity, 

 where there suddenly appears a deep cup, dividing the ter- 

 mination into two knobs. This might at first appear as if bored 

 into by another mollusk, but (1) the excavation is not sideways 

 but from the outer end ; (2) the remains of the animal are 

 fresh within ; (3) the lines of growth on the incrustation dis- 

 play a similar outline. Long. 3Q, kit. 'hi, alt. 'lo. 



Sah. — Mazatlan ; 1 sp. in Spondylus calcifer ; L'pool Col. 

 Tablet 571 contains the specimen. 



175. LlTHOPHAGUS PLTJMULA, Sanl. 



Proc. ZooL Soc. 1844, p. 17. 



Modiola plumida, Hani. Descr. Cat. p. 239. 



Comp. L. Isevigatus, B. M. non Quof/ ^" Gaim. (Cape Upstart, 



Jickes.) 

 Comp. L. ruglfcrus, Bkr. in lit. Mazatlan (teste Cuming) : 



"dlffert a L. plumrda forma et incrustationis indole diversa. 



Margines cardiualis et basalis baud paralleli, pars anterior 



minus inflata est. Latus basale rugis instructum est." Dun- 



ker. Spec. unie. in Mus. Cum. no. 172. 



The species is known by the remarkable character of the 

 incrustation, which for the most part presents the arrange- 

 ment of a feather in lines running out on each side from a 

 midrib which joins the umbo to the posterior end. The in- 

 crustations form beaks beyond the shell, appressed but not 

 prolonged or hollowed within. Its texture is sometimes 

 tolerably solid, sometimes in a branching network. It does 

 not present an organized structure under the microscope, nor 



