IX 



And that difficulty has been strikingly illustrated by the positive 

 withdrawal, by an able naturalist, of at least the Brachiopods 

 and Bryozoans from the true Mollusks, and the combination of 

 them with the Worms. If, then, a deviation from the example of 

 Prof. Huxley and other masters in systematic zoology has been 

 ventured in still retaining the combination of the two groups 

 under the common branch name of Mollusca, it has been because 

 there is still a certain conventional convenience in so doing, and 

 because some members of the lower group (the Brachiopods) are 

 almost always at least by collectors considered in connection 

 with the higher forms. Another and more scientific reason is 

 that at the confines of the lower groups, the hiatus between the 

 two appears disproportionately little compared to that between 

 the other branches, and a stricter series of homologies are 

 traceable between the two. PJiodosoma (Schizascus, St.) of the 

 Tunicates, and the recently described RhaMopleura, Allmann, 

 of the Bryozoans, are especially noticeable in this connection. 

 It may also be added that the difficulty of framing a common 

 diagnosis for the combined types appears to be the result of the 

 diversity of secondary modifications and ramifications, and the 

 extreme specialization of some forms and loss of common primi- 

 tive characters, rather than of the divergence of the two types 

 from a generalized Proto-zoon or aboriginal primordial stock an 

 element necessary to be considered in appreciation of the values 

 of groups. In such cases, the test .must be a series of consecu- 

 tive inductions, and if those can be rigorously established, the 

 truth cannot be far distant, even though an exclusive diagnosis 

 cannot be applied. Care, however, must be taken not to abuse 

 the privilege of combination without exact diagnosis, and the 

 same latitude is not allowable in smaller and subordinate groups 

 as in the more comprehensive. 



CLASSES. 



With regard to the classes of Mollusks, it is only necessary to 

 state that the Pteropods have been considered as a subclass of 

 Gasteropods, and thus retained in one and the same class with 

 the typical members of the latter, in accordance with the views of 

 most American malacologists, and because the hiatus between 



them appears to be much less than that between the Cephalopods 



1 





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