114 NACELLA. 



prove to be a Patina, which the types, seen by me in the U. S. 

 National Museum, resemble as much as they do Nacella. 



Subfamily NACELLIN^E, Thiele. 



The researches of Thiele have demonstrated that there are but 

 two lateral teeth on each side in Nacella, Patinella and Helcioniscus, 

 whilst Patella, Helcion, Patina, etc., possess three on each side. 

 This difference is undoubtedy of considerable value, and I therefore 

 depart from the arrangement adopted in the synopsis of groups on 

 page 79, and consider the forms in my second division " B. One 

 inner lateral tooth on each side anterior " as constituting the sub- 

 family NACELLIN^E. 



The subfamily differs from PatellincK in possessing the character- 

 istic dental formula 3 (i m i)3, and in the shells having a distinctly 

 metallic luster inside ; the genus Patella having the formula of teeth 

 3 ( l ' 212 1 ) 3 and the inside of the shell either transparent and fibrous, or 

 opaque, porcellanous. 



On pages 79, 80, I have divided this group into two subgenera : 



(I) NACELLA, with sections Nacella s. s. and Patinella Dall, and 



(II) HELCIONISCUS Dall. These divisions are used in the same 

 limits by Thiele (I. c.), except that he considers the three as of 

 generic rank, as Dall has already done. 



Genus NACELLA Schumacher, 1817. 



Nacella SCHUM., Essai d'un nou.v. Syst., p. 179. DALL, Amer. 

 Journ. Conch, vi, p. 274, 1871. THIELE, Das Gebiss der Schn. ii, 

 p. 329. not Nacella of CARPENTER, SARS, et al 



The gill-cordon is continuous. 



The foot is encircled by a scalloped epipodial ridge, interrupted in 

 front. 



The dentition is practically the same as in Heleioniscus (q. v.~), 

 differing notably from that of Patella and Helcion-}- Patina. 



The shell has the apex subcentral or anterior, and is characterized 

 by a peculiarly metallic texture, having the central area of the in- 

 terior generally of a red-bronze color. 



The shell and dentition of Nacella approach near to Helcioniscus, 

 but from this and all other Patellidce it is sundered by the presence 

 of a developed epipodial ridge. 



Cape Horn was evidently the birth place of Nacella and Patinella. 

 Thence they have been distributed eastward to the Falkland, New 



