CARBONIFEROUS AGE. 25 



cloxed liy a false deltidium, instead of being ojx'n, and hence they could 

 scarcely have l>cen attached !i\ a peduncle, unless it mav have been during the 

 carl) xt;i-, x nt' growth. Attain, they liavi> tin- beak of tin- ventral valve often 

 niurh more produced, and more or !-> twixted m- distorted as if from ha\ ing been 

 attached hy the substam of the shell. They aKo have tin- dental lamina' of the 

 ventral valve lex-, prominent, and converging under the area towards the beak, 

 instead of extending further within the val\e. The cardinal process of tlic other 

 valve ix likewixe more developed, and the inner socket walls nnieh lexs so. A still 

 more important difference, if it should pro\e to be constant, as seems to be the 

 axe, ix tin- nierelv tiliroiis shell structure in this group, and the tibro-punctate 

 structure in O /////*. 



This genus has also been confounded with Klit*nnl*>n /'//.-, 1'ander (= Orthisina, 

 D'Orb.). but Mr. Davidson hax sliown these two groups to be distinguished by well 

 detined external and internal characters. For instance, in KHtninlttmit&t there is a 

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

 j,,;,,iit>.-< ( Stn i>t<>rli >j iirh nx) the area of the dorsal valve is generally narrower merely 

 rudimentaiv. A^ain, in l\'lifiiinltn!tes the false deltidium covering the fissure of the 

 larger or ventral valve is pierced near the beak by a rounded or oval aperture never 

 seen in ILiiiifii -nnitt'K. The beak of the ventral valve of I\lil<tinlxniit< likewise 

 differs in never being twisted, as we often see in Hemipronitee, and its area is 

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

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

 lateral depressions, and is covered by the false deltidium; while in Ih >ni/> r< >nili A 

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

 differences in the details of the musctdar impressions. 



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

 liiyh 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. Orthixina miiuxniriana and O. Muiin>ir<li<in(i, of 

 Swallow as well as X/ri jifor/ii/iir/nt* t*'r!</, ntnH* and ,V. /-///<///</</////.-, of Nevvlierry, 

 are American examples of this type. /'/<*/////.< ,.,/////* of Kichwald, from the 

 Carboniferous rocks of Russia, and >'////)/<,//<///,<////.. /ctini/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 KKtambonUet. He in- 

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

 distinct, the first of which he called Pronites, and the second 11< n<i/> /-(mites. The 

 tvpical species of his group Pronites (P. adscendent, Pander) being also the type 

 of the including genus Kliiii>iil*,ii!fc*, the name Pronite* must be regarded as merely 

 a xvnonvm of l\Ht,imbonitex, since it would be an absurdity to retain a separate 

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

 this the name Pr<>,,ii,x had been used in 1811 for a genus of birds b\ llligcr. 



The species adscendenn, the type of h'/i/mnbonites, presents all the generic 

 characters of the group to which D'Orbigny subsequently gave the name Ort/ii^iim ; 

 while the typical species of the other supposed subgeuus, J/tiiiij>ni>u'lcn(II.tnin!<lii*, 



4 February, 1864. 



