CARBONIFEROUS AGE. 25 



closed by a false deltidium, instead of being open, and hence they could 

 scarcely have been attached by a peduncle, unless it may have been during the 

 early stages of growth. Again, they have the beak of the ventral valve often 

 much more produced, and more or less twisted or distorted as if from having been 

 attached by the substance of the shell. They also have the dental lamina; of the 

 ventral valve less prominent, and converging under the area towards the beak, 

 instead of extending farther within the valve. The cardinal process of the other 

 valve is likewise more developed, and the inner socket walls much less so. A still 

 more important difference, if it should prove to be constant, as seems to be the 

 case, is the merely fibrous shell structure in this group, and the fibro-punctate 

 structure in Orthis. 



This genus has also been confounded with KUtambonites, Pander (^ OrtJdsina, 

 D'Orb.), but Mr. Davidson has shown these two groups to be distinguished by well 

 defined external and internal characters. Eor instance, in KUtambonites there is a 

 well developed area, provided with a covered fissure in each valve ; while in Hemi- 

 pronites (= Sto'eptorhynclms) the area of the dorsal valve is generally narrow or merely 

 rudimentary. Again, in Klitavihonites the false deltidium covering the fissure of the 

 larger or ventral valve is pierced near the beak by a rounded or oval apertiu'e never 

 seen in Hemipi-onites. The beak of the ventral valve of Kliiamhonites likewise 

 differs in never being tAvisted, as we often see in Hemi'pronites, and its area is 

 generally more inclined towards the front of the shell. In the former group the 

 cardinal process also differs in being formed of a single projection, with two small 

 lateral depressions, and is covered by the false deltidium; while in Hemipronites 

 this process is bilobate and exposed. These groups are likewise distinguished by 

 differences in the details of the muscular impressions. 



There are a few peculiar plicated shells, with a more or less distorted beak and a 

 high triangular area provided with a closed fissure, found in the Coal Measures of 

 Kansas and New Mexico, which appear to form a section of this group, though 

 they may be generically distinct. Orthisina missouriana and 0. Sliumardiana, of 

 Swallow — as well as Strep>torliync]tus oceidentalis and S. pyramidalis of Newberry, 

 are American examples of this type. Productus eximius of Eichwald, from the 

 Carboniferous rocks of Russia, and Streptorhynchus pectmi/ormis, Davidson, from 

 India, also belong to this plicated section. 



This genus was first made known by Pander, in 1830, in the work cited in the 

 synonymy at the head of this description, under the name Klitamhonites. He in- 

 cluded, however, under this name, two groups regarded by him as subgenericaUy 

 distinct, the first of which he called Pronites, and the second Hemipronites. The 

 typical species of his group Pronites {P. adscendens. Pander) being also the type 

 of the including genus Klitamhonites, the name Pronites must be regarded as merely 

 a synonym of Klitamhonites, since it would be an absurdity to retain a separate 

 subgeneric name for the typical species of the including genus. In addition to 

 this, the name Pronites had been used in 1811 for a genus of birds by lUiger. 



The species adscendens, the type of Klitamhonites, presents all the generic 

 characters of the group to which D'Orbigny siibsequently gave the name Orthisina ; 

 while the typical species of the other supposed subgenus, Hemipronites {H.tumidus, 



4 February, 1864. 



