330 PATELLID^. 



a large, simple rachidian tooth with, on each side, two large, 

 simple, transverse laterals, followed by two minute ones, and a 

 large outer lateral with a strong tridentate cusp, outside of which 

 is a single scale-like flat uncinus, bearing an elongated, thickened 

 ridge, but no cusp. 



For this genus Mr. Dall has proposed a new family ; he states 

 that it might be incorporated with Cocculinidse, " were it not for 

 the difi'erences in the branchiae and in its dentition." 



Family PATELLID^. 



Shell wholly external, dish-shaped, with apex anteriorly 

 directed ; animal with two short tentacles, a non-extensible 

 muzzle ; branchiae external or none ; renal and anal aperture 

 situated above the neck, between body and mantle-edge ; no 

 copulatory or external genital organs ; mouth provided with 

 horny jaw, and long radula with numerous peculiar black, 

 opaque teeth, and pellucid or colored plates or bosses ; meta- 

 morphosis of the embryo taking place in the egg, which is fer- 

 tilized in the ovary. 



The Limpets have been verj^ thoroughly studied by Mr. Wm. 

 H. Dall,* who has proposed an elaborate classiflcation of them, 

 including ordinal and subordinal as well as family and generic 

 characters. I have mainly followed Mr. Ball's system in the 

 diagnoses and sequence of the groups, but without giving them 

 the same systematic values : — for example, I have used above his 

 characters of the Order Docoglossa for the family Patellidae, 

 thus making the family more comprehensive than in his sense, 

 and corresponding more nearly in conchological importance with 

 the other families in this work. Similarly Mr. Ball's suborders 

 correspond nearly with ray subfamilies and his family charac- 

 ters are here treated as generic. The order Bocoglossa is, as its 

 name implies, founded upon peculiarities in the arrangement of 

 the lingual dentition (xii, 51), but ali'ead}^ two forms of limpets 

 have been discovered which by their dentition cannot be placed 

 in this order. Cuvier united the Patellidae and Chitonidae in his 

 order Cyclobranchiata, characterized by the arrangement of the 

 gills in a circle surrounding the body, but more recent investiga- 

 tors have ascertained a considerable cliversitj' of gill-arrangement 

 among the limpets, so that this term will no longer apply to 

 them as a whole, although many of them agree with the Chitons 

 in this feature. 



Subfamily LEPE TIN^. 



Animal without branchiee. Embryonic shell spiral. 



* See his papers in Am. Jour. Conch., v, vi, and Proceedings of the 

 National Museum. 



