OF SOUTHERN INDIA. 280 



For some species of which he calls JS. hioculata, d'Orb., the type, he proposed 

 the name Dorh'ujnia, ( ? Orbignia), and a third group of which II. monilifera from 

 Jamaica is the type he calls Bcu-i^ettia (vide Geologist, 1862, p. 375). Dorhignia is 

 said to have the hinge-rib obsolete, but this ajipears to be the case only in some 

 specimens in which it becomes obsolete near the peristome of the shell. Such is also 

 often the case in SphtBriiUtes and RadiolUes. In Barrettia the hinge-rib also becomes 

 obsolete near the aperture, but exists below in the interior of the lower valve, 

 and the outer layer of the shell is reduced to numerous narrow columns of different 

 length, entii*ely cemented by the inner layer of the shell, which besides occupies 

 the whole interior of the valve. I have not seen either S. bioculata or monilifera, 

 but the character referring to the sooner or later disappearance of the hinge-rib near 

 the peristome, pointed out, seems to me to be of little importance, and I have 

 great doubt that it can be used even in a sub-generic sense. The structure and 

 thickness of the various sjDecies of Ilippurites vary enormously, and the way in 

 which a shell has been preserved often alters a good deal its aspect. 



Zittel in his admirable Monograph* of the Gosau Rudistce suggests that two 

 more sections can be distinguished, one represented by II. organisans and the other 

 by II. dilatata. The first has the outer shell layer not much developed, but still 

 more than II. moniliferus and most other Hippurites which are not strongly ribbed. 

 E. dilatata has the anterior muscular impression situated on a laminar jirojection, a 

 character which also appears to be common to many other species of Hippurites. 

 Parts of it are distinctly preserved in our specimen of Hipp, radiosa, Desm. 



4. Tamiosoma, Conrad, 1856. The type of this genus is a very peculiar fossil 

 from the upper miocene deposits of California, T. gregaria, Con. Gabb in the 2nd 

 volume of the Palaeontology of California (pp. 61-63) has very ably discussed the 

 organization of this fossil, and comes to the conclusion that it is most likely a 

 species of the Hippukitid^. The specimens which have up to the present been 

 found resemble the elongated, lower valves of Hipp>u7'ites with a small place of 

 attachment apparently at the thinner or lower end. They are sub-cylindrical 

 with rather thick walls consisting of two or three layers, possessing the same 

 reticulated and striated structure as that of Radiolites, and others. The lower 

 portion of the shell is composed of a large number of irregular chambers or septa 

 which are produced by lateral prolongations of the inner wall. The end is occupied 

 by a large cavity, similar to the ' body-chamber' of Hijjptirites, but no impressions 

 of teeth have as yet been observed in it. The outer surface is longitudinally 

 striated in the type species which grows in clusters, as does, for instance, H. 

 organisans, Defr. 



"When I first saw Conrad's description and figure of this species the irre- 

 gularity of the chambers or septa gave me the impression that the fossil might be 

 a coral, but the more detailed description of Gabb, though it does not altogether 

 remove that impression, — certainly points out aflinities which are very much in 



* Deuk. Akad., Wien, vol. xxv, pt. 2, p. 135. 



3 N 



