XI 



a decidedly retrograde step, and thus morphology was entirely- 

 subordinated to teleology, and even to a degree seldom equalled 

 in recent times ; for the groups enumerated are so very distinct 

 from each other that they have no characters in common except 

 those which they share with others as members of the same class, 

 and the ability to breathe air direct — and even the adaptation for 

 the latter oflQce is alfected by difi'ereut modifications in the seve- 

 ral subclasses. 



The Heteropods, instead of representing a distinct class or 

 subclass, are perhaps scarcely entitled to ordinal rank, but, as 

 their distinctive characters are not entirely adaptive, they have 

 for the present been accredited with it. Besides the Dentalia (So- 

 J.EXOCONCHA), the Chitonidse (Polyplacophora) have been re- 

 moved from the association with the PalelUdse and Acmaeidse, and 

 for the last alone has been retained the ordinal name (Doco- 

 glossa) proposed by Dr. Troschel for all the groups mentioned. 

 It is difficult to understand why the Chitonidse have been so per- 

 sistently associated with Patellidse, except for the reason that 

 after the first discovery of the homologies between the two types, 

 the great differences between them were in a measure lost sight 

 of — a fault common to discoverers of unexpected relationships — 

 and that most others have since been content to accept without 

 active thought the approximation at first suggested. The simi- 

 larity of the nervous system, recently urged in justification, seems 

 to be more superficial than real, and rather the result of adapta- 

 tion to the oval depressed form common to both. Although the 

 author has been the first to limit (in manuscript long ago pre- 

 pared) the order to the families now retained in it, the ordinal 

 name proposed by Dr. Troschel (Docoglossa) being a suggestive 

 one, it has been preferred to a new name.^ 



It need only be added that the orders of Conchifers and of all the 

 Molluscoids are adopted simply as appearing to be the best that 

 have been devised, and not because they are those likely to be 

 ultimately confirmed, at least with precisely their present limits. 



' Mr. W. H. Dall, after an extensive study of the anatomy of members 

 of the group, had also arrived at the same conclusions, and was the first 

 to demonstrate the entire want of affinity therewith of the Gadiuiidee. 



