PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 281 



AVERAGE OF THE SPECIMENS. 



Length of head in total length without caudal (times) 4. 62 



Interorbital area in total length without caudal (times) 43 



Snout in total length without caudal (times) 17 



Upper jaw in total length without caudal (times) 14. 05 



Mandible in total length without caudal (times) 11 



Distance of dorsal from snout in total length without caudal (times) 4. 73 



Base of dorsal in total length without caudal (times) 1. 2(5 



Distance of anal from snout in total length without caudal (times) 2. 17 



Base of anal in total length without caudal (times) 1. 84 



Distance of pectoral from snout in total length without caudal (times) 4. 51 



Length of pectoral in total length without caudal (times) 5. 95 



Distance of ventral from snout in total leugth without caudal (times) 4. 79 



Length of ventral in total length without caudal (times) 13. 74 



Branchiostegals VI 



Dorsal rays 48-50 



Anal rays 33-37 



Caudal rays 21-22 



Pectoral rays 15 



Ventral rays 3 



U. S. National Museum, Washington, December 4, 1878. 



K 



REPORT ON THE LIMPETS AND CHITONS OF THE ALASKAN AND 

 ARCTIC REGIONS, WITH DESCRIPTIONS OF GENERA AND SPE- 

 CIES BELIEVED TO RE NEW. 



By W. II. DALL. 



The following report has been drawn up chiefly from material collected 

 in Alaska from 1865 to 1874 inclusive, but includes references to the 

 few Arctic or northern species which are not common to Alaskan waters. 



The northwest coast of America, which I have already stated I have 

 reason to think is the original center of distribution for the group of Boco- 

 glossaj at least of the littoral forms, is unquestionably the richest field 

 where these animals may be found. This is true not only in regard to 

 the number of species, but also in regard to' the number of peculiar and 

 remarkable forms of genera and subgenera; in one sense, the develope- 

 ment and specialization of the soft parts, even at the expense of the 

 shelly envelope, is a test of relative rank in restricted groups. Hence 

 it may not be erroneous to regard the gigantic Cryptochiton as represent- 

 ing the highest developement of the group, though belonging in the 

 section of Irregular Chitons; especially as paleontological evidence 

 shows part of the section of Kegular Chitons to represent the Chitons of 

 paleozoic times and embryonic structure. As was pointed out in my 

 previous paper on the phylogeny of the Docoglossa, the embryonic 

 types, represented by Lepeta and Cryptobrancliia among the Limpets, arc 

 represented on the £T. W. coast by a larger number of species and by 

 larger individuals than in any other region ; so the embryonic types of 

 Chitoniclcc in the same district are here to be found more largely repre- 



