330 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



giilosis, valde separatis; subgrundis spongiosis, parum extantibns; siim 

 alto, lato, planato, spongioso, hand laminato; pagina interna callosa; 

 zona fusca, minntissime grannlosa, inter grannlas setis pellucidis mini- 

 mis line et illuc decurreutibns. Lon. 25, Lat. 16 mm. (Cpr. IMS.) 



Hah. — Columbian Arcbipelago, probably reacliing the southern bor- 

 ders of Alaska, and southward to Magdalena Bay, Lower California. 

 Forty specimens examined. 



This species having been originally described from imperfect speci- 

 mens, I insert Dr. Carpenter's amended diagnosis. It has not occurred 

 in our collections, but being abundant in the Yancouver region, doubt- 

 less occurs in Southeastern Alaska. It is an aberrant species, and at 

 some time may require to be separated from the genus to which Dr. Car- 

 penter and myself have x^rovisionally referred it. 



Chagtopleura Nuttallii. 



Chiton mittaUU Cpr. P. Z. S. 1855, p. 231. 



Track ydennon Nuttallii Cpr. Supj)l. Eep. Br. Assoc. 1863, p. 649, 



C. t. mucrone satis planato; intus v. post. 11-, ant. 8-, ceutr. 1-fissata; 

 aliter ut in C. Hartwegii formata. 



Hab. — With the last, also probably in Alaska. 



All the specimens examined appear to differ from C. Sartwegil in the 

 broad non-sweUing valves, squared at the sides, and not beaked or waved. 

 It may yet prove merely a variety. The characters of the mantle and 

 interior are aberrant, as in the last species. 



Genus ISCHNOCniTO:N". 



IsclmocUton Gray ^*, P. Z. S. 1847, pp. 123-7. 

 Lepidoplcurus Ad. Gen. Eec. Moll, i, 471, 1854. 



Testa tenuior ; lam. insert, regulares, acutte, nee pectinatae nee serra- 

 tai; subgrundiB majores; sinus i)lerumque laivis; zona squamosa, squa- 

 misplerumquestriatis; branchiae elongatie. (Q)r.) Type J. longicymha 

 Quoy. 



The main character of this genus, which includes by far the largest 

 number of species of any single group of Chitons, consists in the row of 

 sharp smooth insertion-teeth, surrounded by more or less projecting 

 eaves, as first described by Di\ Carpenter in the Mazatlan Catalogue 

 (p. 191), and in the scaly girdle. Dr. Carpenter has divided the group 

 by its minor characters into the following subgenera: 



1. Stcnoplax Cpr C. limaeiformis Sowerby. 



Body elougatc. Scales elougate, chaflfy, striated, irregular, aud crowded. 



2. Stcnoradsia Cpr C. magdaJcnensis Hinds. 



Lilie Stcnoplax, -with numerous side-slits. 



3. Iscluioj)lax Cpr C. pcctinatns Sowerby. 



Like Stcnoplax, but with occasional large scales rising above the rest, aud a mul- 

 titude of sliort striated bristles. Mucro raised, subposterior. 



