330 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



gulosis.valde separatism subgniudis spongiosis, parmn extantibus; sinu 

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

 zona fiisca, minutissime granulosa, inter grannlas setis pellucidis mini- 

 mis buc et illnc decurrentibns. Lon, 25, Lat. IG mm, (Cpr. MS.) 



Hah. — Columbian Arcbipelago, probably reacbing tbe 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. 



Chaetopleura Nuttallii. 



CUion XuttalUi Cpr. P. Z. S. 1855, p. 231. 



Trachydermon XuttalUi Cpr. Suppl. Eep. Br. Assoc. 1863, p. 649. 



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

 alitor ut in C. Hartwegii formata. 



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



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



hclcnocldton Gray ^*, P. Z. S. 1847, pp. 126-7. 

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



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

 tae; subgrundae majores; sinus plerumque Isevis; zona squamosa, squa- 

 misplerumque striatis; branchipe elongatie. (Cpr.) Type J. longicyniba 

 Quoy. 



The main character of this genus, which mcludes 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. Cai-penter in the INIazatlan Catalogue 

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

 by its minor characters into the follo^ving subgenera: 



1. Stenoplax Cpv - C. Umacifoiinis Sowerby. 



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



2. Stenoradsia Cpr C. magdalenensis Hin^s. 



Like Stenoplax, with numerous side-slits. 



3. Ischnoplax Cpr C. pectinatus Sowerby. 



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

 titude of short striated bristles. Mncro raised, subposterior. 



