298 THE r EARLY FRESB-WATER MUSSELS— SIMPSON. vol.xviii. 



Unionidne.' The Castalias, Castalinas and Hyrias have the radial beak 

 sculpture whicli is found on every species of South American Unio^ but 

 on none of the other Naiades. The hinge teeth consist of cardinals 

 and laterals, the former being more divided than is usual in Unio., 

 though there are some species in the latter genus which have the car- 

 dinals separated into several parts. The laterals are Unionoid, but are 

 more or less vertically striated in CastaUa and Castalma, and some- 

 times, to a certain extent, in Hyria. This latter character, however, is 

 not generic or even specific. The hinge teeth in the bivalves were 

 undoubtedly developed in order to lock the valves of such species as 

 were subject to shock, and prevent them from being twisted out of 

 place. I believe it will be found that in most, if not all cases where they 

 are needed, the shell never opens so far but what they lock one valve with 

 the other. The mantle is carried as a thin, tough, elastic sheet between 

 the hinge plates and over the teeth in the Naiades, and it will be 

 readily seen that any unusual roughening, such as the development of 

 granules or vertical striation, would render them much less liable to 

 slip than if they were smooth. Hence, in many solid-shelled Unionids, 

 especially in elongated species, the character of vertical teeth stria- 

 tion will be found. It is especially developed in many of the heavy Chi- 

 nese Unios, and I have noticed it in Unio parallelopipedon of South 

 America, in Unio shepardianus, ligamentinus, crassus, luteolus, anodon- 

 toides, and otliers, of the United States.^ 



Unio kraussi, Lea, of Surinam, of which the type is in the National 

 Museum (No. 84379), seems to stand about midway, conchologically, 

 between Unio and CastaUa^ but in a ditterent direction from Castalina. 

 It has the strong radial beak sculpture of Castalia, especially near the 

 posterior ridge, where it extends more than one-third of the distance 

 from the beaks to the periphery. It is much inflated, and has a form 

 more circular than that of Castalia, a brown epidermis and strong con- 

 centric sculpture. The teeth stand about midway between those of 



'Ihering believes that Hyria will be found to vary in the character of its mantle 

 openings in the same waj' that Ca8<rt?ma and Ca«<aZia do. (Zool. Anzeiger, Nos. 380 

 and 381, 1891-92, p. 5.) 



-The characters of the teeth of the four genera Unio, Hyria, Castalia and Castalina, 

 are very variable. Unio charruanus, d'Orbigny, has about 4 strong cardinals and sev- 

 eral minor teeth in each valve, besides the ordinary laterals, which, with quite a 

 number of not closely related species from Brazil, show traces of vertical striation. 

 Unio acutirostris, Lea, from southern South America, has about 12 denticles in the 

 cardinal area of each valve. In the younger shells there are usually the ordinary 

 compressed cardinals, one in the left valve and two in the right, and as the specimens 

 become adult they split up and assume a very difiereut appearance. Unio pata- 

 gonicus, d'Orbigny, shows this transition finely. In Unio gibbosus, Barnes, of the 

 United States, the laterals are quite often somewhat vertically striated, and some- 

 times have oblique striai pointing anteriorly or posteriorly. Specimens of Castalina 

 martenai, v. Ihering, in the National Museum (No. 125736), plainly show both vertical 

 and oblique strife on the laterals in the same hinge, the oblique lines being finer 

 and partly laid over the vertical ridges. 



