1895. 



PROCEEDINGS OF THE NATIONAL MUSEUM. 315 



not stand, as it was preoccupied in Vermes by Savigny in 1817. The 

 name Frisodou of Schumacber, which is sometimes given to this genus, 

 will have to be used, I think, for the symphynote forms belonging to 

 the group commonly called Hyria. The systematic position and rela- 

 tionships of this group have been discussed in this paper under the head 

 of o-eneral classification, and the genus undoubtedly should be placed 

 witb the Unionida\ According to Orbigny,^ the animals examnied by 

 him had the mantle free the whole length, except in the anal region, 

 where it was developed into two short distinct tubes, of which the bran- 

 chial was the larger and furnished with cirri. The branchiic were large 

 and slightly unequal, and the rounded palpi were very large. 



The Adams Brothers state that the outer gill is united to the mantle 

 as far as its extremity, which does not agree with the observations of 

 V Ihering. According to this observer, Tetraplodon quadrilatera has 

 a'rouude^Ii triangular glochidium without hooks, the embryos being 

 developed in the inner gills. 



Genus CASTALINA, v. Ihering.- 



This genus, of which certain characters have already been discussed, 

 was founded by its author for a few species of South American Naiades 

 which have a somewhat triangular outline and appear to stand about 

 midway between Unio and Tetraplodon. The fact, as v. Ihering 

 declares that shells of certain species of the group may contain animals 

 with an open mantle which are perfect Uiiios, and that others have soft 

 parts with closed siphons, and are therefore Tetraplodons, shows that 

 there is a very close relation between Vnio and Tetraplodon, and that 

 this is a transition group, which, from the characters of the animal alone, 

 would not be worthy of generic rank; but the shells are sutficiently 

 distinct from both the above genera to be separated without great 



difficulty. 



Their cardinals are much like those of Umo, only more numerous, 

 and the laterals often have traces of vertical or oblique striation, while 

 the posterior ridge is less strong than it is in Castalia, and the shells 

 are more compressed. 



Genus PRISODON, Schumacher.^* 



The genus Prisodon included under Section A, P. ohUqtius, Schu- 

 macher, which is a species that has since been placed in Lamarck's 

 genus Hyria; and under Section B, P. truncatus, Schumacher, a mem- 

 ber of another genus, which is now more commonly put in Lamarck's 

 Castalia. The excellent figures and descriptions of these species leave 

 no doubt that the above conclusion is correct, while the generic descrip- 

 tion fairly covers the first sp ecies, and it seems to me, notwithstanding 



Voy. Am. M^r., p. 597. 

 ^Zool. Anzeiger, 1891, p. 478. 

 3E88ai Nouv. Systenie des Habit, vers Testac^^s, 1817, p. 138. pi. xi, tig. -. 



