18 THE NAUTILUS. 



intended to be a representative series of the fauna of the river at 

 the place, not a single specimen could be satisfactorily referred to 

 " trigona." It would seem as though it did not extend up the river 

 as far as that place. 



It is unquestionable that the form I have identified as the undatus 

 of Barnes has for the last half-century been uniformly considered to 

 be the trigonus of Lea. 



It is also true that prior to 1850, Lea's species was considered to 

 be the same as Barnes's by a very large and respectable element 

 among the conchologists of that time. 



It is evident, also, that Dr. Lea had considerable difficulty in en- 

 forcing the adoption of his disposition of undatus (Syn. 4th, Ed., p. 

 38 n ; Rectification 1st Ed. p. 15) at that time. 



In considering the question de novo it must be admitted at once 

 that the typical forms of the two " species " are not exactly the same. 



Conrad in a note to his last Synopsis (1853), in which he considers 

 the two forms to be distinct species states the difference aptly: it 

 (undatus^) " is much more ventricose anteriorly and over the umbo 

 than trigonus ; has more elevated beaks and is very inequilateral, 

 whilst the latter is nearly equilateral." Dr. Lea remarks (Syn. 4th 

 Ed. p. 38 n 3) that trigonus is always more angular on the umbonial 

 slope and the undulations at the tip of the beaks differ." 



This comparison does not apply to the true undatus of Barnes, but 

 is correct in the first item when applied to obliquus, with which he 

 considered Barnes's species to be synonymous. I have not been able 

 to ascertain the beak characters of obliqua from the material at my 

 disposal. 



In considering Lea's conception of his species and whether he con- 

 sidered it to include the form believed to be the real undatus of 

 Barnes, it may be of service to note his treatment of both species 

 before and after his identification of undatus with obliquus in 1832. 



In his original description of trigonus (Obs. I, p. 121) he remarks 

 that his species belongs to the group of species which are known as 

 mytiloides Raf. and has been considered a variety of that species, but 

 that he believes that the group may be divided into four species, 

 mytiloides Raf., undatus Bar., pyramidatus Lea, and trigonus Lea. 



It is to be noted that this was before he had identified undatus with 

 obliquus, but tends to show that he even then identified Barnes's spe- 

 cies with the form, which he subsequently declared to be obliquus 



