﻿254 NKI'IIKON'AIAS 



ened on the edg-e. which is often dark colored ; anal and bran- 

 chial openings larc^c and well furnished with ])a])illae. Noth- 

 mg is known of the niarsii])iiini of any members of this j^roup, 

 the animals examined probably bein)^ males ; but it is most 

 likely that when gravid the ovules will be found in distinctly 

 marked ovisacs in the posterior part of the outer gill, some- 

 thing as in Lampsilis. 



Type, Unio plicatiiliis Charpentier. 



The g^eniis Acphronaias, as diagnosed in the Synopsis and 

 the present work, consists of some twenty or more species 

 belonging to Mexico and Central America, with two or three 

 species extending into the isthmian part of Colombia and 

 possibly one or two reaching extreme southern Texas. I 

 regret that I have not been able to examine g'ravid females of 

 any of the s])ecies, but I am fully convinced that while the 

 group has some characters allying it to Unio, such as the biang- 

 ulation of the ])osterior end, there generally being a distinct, 

 double ])osterior ridge, and the arcuation of many old male 

 shells, yet it is most closely related to Lampsilis. 



I have been fortunate enough to examine shells of nearly 

 all the species and these with the figures of Sowerby, Kuster, 

 Fischer and Crosse and von Martens have shown me that in 

 nearly every species the shells are distinctly diinorpliic. That 

 of the male is either incurved, straight, or very slightly 

 rounded out on the base with the ang-le of the lower posterior 

 ridge on the base line, while that of the female has a more or 

 less decided marsupial swelling, is never arcuate, and has 

 the lower point of the posterior ridge above the base line. 



It seems to me that all the authors, w^ho have written on 

 the Naiades of this region have either totally failed to note 

 that a majority of them have shells of two forms or have not 

 given the fact suflficient weight, and in several cases the male 

 shell has received one name and the female another — as is 

 the case in Lampsilis and Tnincilla. In some of the species, 

 notablv U. gimdlachi and medclliniis. the marsupial swelling 



