PliOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 281 



AVERAGE OF THE SPECIMENS. 



Leugtli of head iu totallcngtli without caudal (times) 4.GiJ 



luterorbital area in total length without caudal (times) 43 



Snout iu total length without caudal (times) 17 



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



Mandible in total length without caudal (times) 11 



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



Base of dorsal in total length without caudal (times) 1. 26 



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 iu total length 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, Waslnngton, December 4, 1878. 



REPORT 0> TIEIi: L.I.nPI<:T!<i AND CHITONS OF THG AIiASKAIV AND 

 ARCTIC REGIONS, WITH DESCRIPTIONS OF OENERA AND SPE- 

 CIES BELIEVED TO BE NEW. 



By W. H. DALI.. 



The followiug report has been drawn up chiedy from material collected 

 iu Alaska from 1805 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- 

 glossa, 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 an<l 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 Crijpfoehiton as represent- 

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

 section of Irregular Chitons; especially as paleontological e^adence 

 shows })art of the .section of Ivcgular (^liitons to represent the Chitons of 

 paleozoic times and eml)ryonic structure. As was i^ointed out in my 

 previous paper on the phylogeny of the Docoglossa, the embryonic 

 types, represented by Lcpetn and Cri/pfohraueJila among tlio Limpets, are 

 represente<l on the N. W, coast by a larger number of species and by 

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

 Chitonulfc in the same district are hen^ to bo found more largely repre- 



