OF CONCHOLOGY. 255 



angtiJaris" to the compressed variety only; it is doubtful, 

 however, if such a course wouhi be admissible, as every transition 

 in form can be observed in a very few specimens. 



COLLISELLA ATRATA, Cpr. Plate 14, fig. 15, 15a. 



Acmcea (? var.) atrata, Cpr., An. Nat. Hist. 3d ser. xiii, p. 

 474, 1864. 



Mantle edge nearly smooth, narrow ; head small ; tentacles 

 very short and stout ; gill small, broad and short; muzzle small, 

 somewhat produced. General hue yellowish. Intermediate 

 between discors, PhiL, and Jloccata, Rve. (Cpr. loc. cit.), all being 

 probable varieties of one species. Cape St. Lucas, Acapulco. 



COLLISELLA PEDICULUS, Phil. Plate 15, fig. 16. 



Patella pediciilus, Phil., Zeitschr. f lir Mai. 1846, p. 21, No. 8. 



Cpr., Maz. Cat. p. 200, No. 260. 

 Patella corrugata, Rve., Conch. Icon. pi. 40, f. 132. 



Animal small in proportion to the size of the shell ; mantle 

 margin nearly smooth, puckered to fit the angles of the ribs; 

 head small, short; muzzle moderate; tentacles very short, stout, 

 pointed. Gill wider than long, triangular, short ; hood large 

 and produced. 



It was with some surprise that I observed the well marked 

 gill on the neck of this species, as the shell characters were 

 essentially those which have been usually regarded as patelloid. 

 But the present case and that of many other species, afford good 

 evidence of the worthlessness of the shell characters in this group 

 as indications of afiinity. 



The young of this species is indistinguishable from " Patella" 

 discors, jun. In the adult discors, however, the ribs become 

 evanescent and usually disappear entirely. The sculpture is 

 identical in both, barring the ribs, as is the apex in most cases. 

 Still the aspect of the adult is generally sufficiently distinct to 

 be readily recognizable, so that we can well afford, in the ab- 

 sence of any knowledge of the animal of discors, to consider the 

 two forms distinct. The shells of pediculus, according to Dr. 

 Carpenter, are much like young Ancistromesus Mexicanus ; the 

 animals, however, belong in different families. The habitat of 

 the present species is on the Mexican coast from Acapulco to 

 Mazatlan. 



COLLISELLA SUBRUGOSA, D'Orb. Plate 14, fig. 14. 



Acmoia subrugosa, D'Orb., Voy. Amer. Merid. vol. v, p. 479, 

 No. 442, 1847. 



