Occasional Papers of the Museum of Zoology 29 



smooth forms without mentioning that they differed from the 

 main description in this character, it appears hkely that he 

 did not intend to describe the surface. However, if he did, 

 there is intrinsically no more reason for attributing folds to 

 his form (a) than to (b) and (c) in interpreting his defini- 

 tions. He did not designate either of the forms as a "Van," 

 as he was accustomed to do where the divergence was thought 

 by him to be considerable. Any one of the included species 

 could be selected to bear the name "crassidens" in a restricted 

 sense, but preferably either form (a) or (b) which he pos- 

 sessed. A method of dividing species has been provided in 

 the International Rules of Nomenclature.^ 



"Lea was the first to learn what Lamarck's species was from 

 an examination of the original specimens (Obs. I, p. 199; H, 

 p. 125). He definitely restricted crassidens to Lamarck's form 

 (b), which he stated to be the Unto cuneatus Barnes. Whether 

 this action was generous or best may be questioned, but he 

 undoubtedly had the right to do so under the rules, which 

 provide that 'such designation is not subject to change.' 



"Lamarck's description, with those of forms (a) and (b), 

 are ambiguous. Experts differ as to what parts of it are intel- 

 ligible without knowledge of the types. But, excluding the 

 reference to Say's Unio crassus, which Lamarck did not rec- 

 ognize as his own U. ligameniina, it cannot fairly be 

 claimed that one of the forms included in crassidens is more 

 recognizable than another. Unless the species is to be thrown 



^ Art. 31. The division of a species into two or more restricted 

 species is subject to the same rules as the division of a genus, etc. 

 {Rep. Zool. Nomencl., Proc. 7th, Internat. Zool. Congr., 1912, p. 47). 



In the division of a genus it is held that "If an author in publish- 

 ing a genus with two or more valid species failed to designate or indi- 

 cate its type, any subsequent author may select the type, and such 

 designation is not subject to change" {op. a't., p. 46, Ilg). 



