CLASSIFICATION OF THE CEPHALOCHORDA. 



283 



differences in myotome formulae are very marked indeed, 

 far more so than in any of the three above species. In 

 fact, the difference between the extreme myotome formulae 

 of A. bassanum is more decided than that between the 

 first and last formulae of the last given table, as the 

 following list shows. 



A. hassanum- 



Pre-atr. 



Pre -anal. 



Post-anal. 



Total. 



Authority. 



44 



13 



18 



75 



Gunther. 



43 



15 



17 



75 



Gunther. 



45 



14 



17 



76 



Gunther. 



43 



16 



12 



71 



Kirkaldy. 



44 



14 



17 



75 



Kirkaldy. 



45 



17 



15 



77 



Kirkaldy. 



45 



16 



14 



75 



Kirkaldy. 



On the above evidence it seems best to place B. caribbceum 

 and B. behheri under the species B. lanceolatum as 

 varieties only, for the differences cannot be regarded as 

 of specific value. There is a danger of depending on 

 single characters and some observers have kept too closely 

 to the arrangement and number of the myotonies alone, 

 have not allowed for variation in the species, and have 

 ignored other characters entirely. Sundevall made the 

 limits of myotome variation too narrow, and later 

 observers have not dared to extend them, though the 

 examination of a number of specimens of each species 

 shows how necessary this is. 



Consider now the three other species, B. calif or niense, 

 B. capense and B. elongatum. 



B. elongatum was described in 1852 by Sundevall. The 

 description was very vague and unsatisfactory, and the 

 only valuable characteristic given is the myotome 



