2 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



submediano, apicibus proniinentibus; areis lateralis iuconspicuis ; tota 

 superficie quincuncialiter minute reticulata; intus, valv. ant. 16-, post. 

 11-, centr. 2-fissatis; dent, parvis, perspougiosis, late separatis; snb- 

 grundis spongiosis, curtis ; sinu parvo ; zona squamulis minutis obsita. 

 Lon. C, lat. 3 mm . 

 Hab. — Western Aleutians, near low- water mark; Dall. 



This bears no marked resemblance to any of the other Alaskan 

 species. 



Genus TONICELLA Cpr. 



Type Chiton marmoreus Fabr. 



Tonicella saccharina Dall, n. s. 



T. t. parva, oblonga, tota superficie saccharina, rufo et albescente 

 picta; mucrone submediano, inconspicuo; areis lateralis inconspicue ele- 

 vatis, albesceutis; areis dorsualissanguinosis, oeque quincuncialiter lente 

 reticularis; v. ant. 10-11-, v. post. 8-10-, v. centr. 1 fissatis ; dent, par- 

 vis spongiosis, sinu parvo; subgrundis spongiosis, mediocris; zona, 

 coriacea ut in ToniceUcv aliis : branchiae rnedise. Lon. G.5, lat. 4 mm . 



Hab. — Aleutian Islands, three to thirteen fathoms; Dall. 



This species has the lustre of rock-candy, and is well marked by the 

 contrast of the white lateral with the red dorsal areas. 



Genus SCHIZOPLAX Dall. 



Testa et zona Tonicrilcc simulans; valvse centrales sulco jugali medi- 

 ano, antico argute incissBj branchiae subambieutes. 



Type Chiton Brandtii Mitld. 



For this remarkable Alaskan form, distinguished from all other Chi- 

 tons by its slit central valves, I propose to adopt a name suggested 

 by Dr. Carpenter, who, on Middeudorf's figures, had intended to propose 

 it as a subgenus of Tonicella. The specimens obtained by my party 

 seem to be the first found since the original ones were obtained. A 

 careful examination of the soft parts shows that in dentition and some 

 other details sufficient basis for generic separation is found, confirming 

 the testimony of the valve characters. The sulcus is usually filled by 

 a horny or cartilaginous deposit. 



It may be added here that investigation of the characters of the 

 radnla in numerous species and genera of Chitons in the National Mu- 

 seum shows a very remarkable uniformity of dentition. No larger 

 groups than genera are indicated in the whole order, which, it appears, 

 can hardly comprise more than one family; and it is doubtful if this can 

 be divided into subfamilies by any characters yet elucidated. 



The dentition in all species examined has the formula G • 2 • 1 • 2 ■ G, or 



n * . Of the teeth, the rhachidian appears always simply cusped ; 



