2 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



submediano, apicibus promineutibus; areis lateralis inconspicuis ; tota 

 superficie quincuncialiter miuute reticuLita; ^/l^MS, valv. aut. 1G-, post. 

 11-, centr. 2-fissatis; Jent. parvis, perspoiigiosis, late separatis; siib- 

 gruudis spongiosis, cuxtis; sinii parvo; 2^ow« squamulis minutis obsita. 

 Lon G, lut. 3'^'". 



Sab. — Western Aleutiaus, near low- water mark; Dall. 



This bears do marked resetablance to any of the other Alaskan 

 species. 



Genus TONIOELLA Cpr. 



Type Chiloii marmoreus Fabr. 



Tonicella saccharina Dall, n. s. 



T. t. par\ a, oblouga, tota superficie saccharina, rufo et albescente 

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

 vatis, albescentis; areis dorsualissanguinosis, seque quincuncialiter lente 

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

 vis spongiosis, sinu parvo; subgrundis spongiosis, mediocris; zon^ 

 coriacea ut in TonicellcG aliis : branchiae mediae. Lon. 6.5, lat. 4™™. 



Hah. — 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 SOHIZOPLAX Dall. 



Testa et zona Tonicellce simulans; valvae centrales sulco jugali medi- 

 ano, antico argute incisae ; branchiae subambientes. 



Type Chiton Brandtii Midd. 



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 sufQcient 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 

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

 seum sliows a very remarkable uniformity of dentition. No larger 

 groups than gpnera 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 al! species examined has the formula 6 • 2 • 1 • 2 • 6, or 



^ -. Of the teeth, the rhachidian appears always simply cusped ; 



