MAZATLAN UNIVALVES 201 



young of P. Mexicana it lias much closer analogies. The 

 largest specimens of P. pecliculus however do not at all run 

 into the smallest of P. Mexicana. They have all the appear- 

 ance of being old shells, with the margin narrow and the 

 shape long and irregular ; while P. Mexicana, as it is traced 

 upwards, displays a very wide semitranslucent margin, and a 

 broad regular shape, with the ribs not rounded and prominent 

 but simply giving an angular form to the shell. To settle the 

 point satisfactorily will require a series of P. Mexicana from 

 its earliest stage, as well as a knowledge of the animals. Of 

 this species I have carefully examined nearly 300 specimens. 

 The youngest shell that has its ribs developed and clearly 

 belongs to this species is ' 17 long. Tablet 912 contains two 

 smaller ones without distinct ribs, which probably belong to it* 

 Even when very young, they arc almost always incriisted with 

 corallinous matter. The largest specimen measures long. 1*3, 

 lat. 1', alt. *4. The ordinary size however is very much smaller 

 and natter. 

 Hob. Mazatlan, Philippi. Acapulco, Reeve. Mazatlan ; not 



uncommon ; L'pool fy Havre Coll. 



Tablet 913 contains 3 young specimens. 914, 3 sp. ordinary 

 state. 915, 3 do. ribs well developed. 916, 3 do. light tortoise- 

 shell within. 917, 3 do. dark tortoiseshell. 918, 3 do. broad 

 variety. 919, 4 do. ribs numerous. 920, 4 do. elongated, ribs 

 small. 921, 1 sp. abnormally elongated, with very small ribs. 

 922, Patella discors, with place of attachment of P. pediculus. 



I 261. PATELLA DISCOES, Phil. 



Abbild. und Besch. Condi, pi. 2, f. 6. Rve. Condi. Ic. sp. 78, 



pi. 29, f. 78, a, b. 



The peculiar characteristic of this species, viz. the crumpling 

 near the umbo, is very seldom seen without careful cleaning, 

 as the shell is almost always thickly coated with vegetable 

 corallinous deposits, algae, &c. The surface is extremely finely 

 ribbed throughout, but at a little distance from the black apex 

 the shell is gathered into irregular undulations, about 15 (more 

 or less) which soon disappear, and leave the shell henceforth of 

 a regular growth. It goes through all the changes common to 

 limpets, being sometimes high, sometimes depressed ; some- 

 times suborbicular, generally oval. The margin is extremely 

 finely crenated, and is generally more or less dotted with black. 

 The prevailing colour is a greenish white, with more or less 



