104 



THE NAUTILUS. 

 ON ACANTHOPLEURA AND ITS SUBGENERA. 



BY H. A. PILSBRY 



111 studying the species of this abundant group of Chitons, the 

 writer found the scheme of subgenera adopted by the late Dr, Car- 

 penter and published by Dall ^ to be in several respects both insuffi- 

 cient and faulty. These defects are partly due to the fact that 

 Carpenter's studies on this portion of the Chitons were left incom- 

 plete by his untimely death, and partly to his rather lax usage in 

 questions of nomenclature. 



Acanthopleura, as a whole, is much more closely allied to Tonicia 

 than to any other genus. It has no close alliance with the other 

 genera grouped by Carpenter in " Acanthoidea." It agrees 

 with Tonicia in having certain areas on the valves studded 

 with minute eyes, which possess a crystalline lens, and are not 

 especially different from lower mollusk eyes generally. Acantho- 

 pleura differs from Tonicia in having these eyes scattered about the 

 bases of the tubercles on the shell, instead of being arranged in 

 radiating series as in Tonicia. The eyes are visible under a good 

 hand lens as minute shining black sunken dots or transparent jewel- 

 like raised drops. 



Carpenter supposed the West Indian species, Ch. piceus, to be the 

 type of Acanthopleura ; but in Guilding's original paper no species 

 of that group of forms is mentioned by name. A critical review of 

 the subject compels us to accept Ch. spinosus Brug. as the type of 

 Acanthopleura. The subgenera will stand as follows: 



(1) Acanthopleura Guild., (re^^tricted.) 



Valves somewhat immersed ; posterior valve having a very long 

 insertion-plate, cut into numerous teeth by short slits; intermediate 

 valves with one or two slits ; sinus smooth ; girdle covered with 

 long spines. Type Ch. spinosus Brug. Francisia Cpr. is a 

 synonym. 



The immersion of the valves gives the tail-valve, when detached, 

 the appearance of that oi Katherina tunicata. 



(2) Maugeria Gray, (restricted.) 



Posterior valve having the pectinated insertion j)late, cut into 

 numerous teeth by slits similar to those of the head-valve. Median 



iProc. U. S. Nat. Mus. 1881, p. 284. 



