330 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



gulosis, valde separatism subgrundis spongiosis, parum extantibus; sinu 

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

 zona fusca, minntissime granulosa, inter granulas setis pellucidis mini- 

 mis hue et illuc decurrentibus. Lon. 25, Lat. 16 mm. (Cpr. MS.) 



Hab. — Columbian Archipelago, probably reaching 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 Vancouver 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 provisionally referred it. 



Chsetopleura Nuttallii. 



CMion Nuttallii Cpr. P. Z. S. 1855, p. 231. 



Trachydermon Nuttallii Cpr. Suppl. Rep. Br. Assoc. 1863, p. 649. 

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

 aliter ut in C. Hartwegii formata. 

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



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

 broad non-swelling 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 ISCHXOCHITOK 



IschnocUton Gray $*, P. Z. S. 1847, pp. 125-7. 

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



Testa tenuior ; lam. insert, regulares, acuta?, nec pectinate nec serra- 

 te; subgrundae majoresj sinus plerumque la? vis ; zona squamosa, squa- 

 mis plcrumque striatis; branchise elongate. (Cpr.) Type J. longicymba 

 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 Dr. Carpenter in the Mazatlan Catalogue 

 (p. 194), and in the scaly girdle. Dr. Carpenter has divided the group 

 by its minor characters into the following subgenera: 



1. Stcnoplax Cpr C. limaciformis Sowerby. 



Body elongate. Scales elongate, chaffy, striated, irregular, and crowded. 



2. Stenoradsia Cpr C. magdalcncnsis Hinds. 



Like Stcnoplax, with numerous side-slits. 



3. Ischnoplax Cpr C. pectinatus Sowerby. 



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

 titude of short striated bristles. Mucro raised, siUmosterior. 



