MAZATLAN BIVALVES 125 



ventrali incurvo ; epidermide fused, rugis concentricis parte 

 posterior e instruct d ; incrustatione in parte posticd solidd, in 

 lineas duas ab umbonibus decurrente,strussagittiformibusincon- 

 spicuis ad marginem directis ; in rostra solida prolongatd, 

 appressa, maxima parte non excavata sed ad 

 apicem alte effossa, quasi caly cem adkibentia. 



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 layer over the posterior part, 

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

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

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

 The incrusting beaks are appressed, as in L. attenuatus, 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. '36, lat. '14, alt. '15. 

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



Tablet 571 contains the specimen. 



-^ 175. LlTHOPHAGTJS PLUMULA, HanL 



Proc. Zool. Soc. 1844, p. 17. 



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



Comp. L. Is&vigatus, JB. M. non Quoy fy Gavin. (Cape Upstart, 



Juices.) 

 Comp. L. rugiferus, DJcr. in lit. Mazatlan (teste Cuming) : 



"differt a L. plumula forma et incrustationis indole diversa. 



Margines cardinalis et basalis haud paralleli, pars anterior 



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



Jeer. Spec. unic. 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 



