C0ITC3XF!REi.i 44 J 



Geoicgical Society of London, In ali the genera the ^sliell con- 

 sists of tJu^ee layers, but the outermost, which is thin and com- 

 pact, is often destroyed by the weathering of the specimens. 

 The principal layer in the lower valve of the Eiixpurite is not 

 really vei^ different from the upper valve in structure; the 

 laminae are corrugated, leaving irregular pores, or tubes, parallel 

 with the long axis of the shell, and often visible on the rim. 

 The umbo of the upper valve of the Badiolite is marginal in the 

 young shell. (Q. J. Geol. Soc, vol. xi. p. 40.) 



They are the most problematic of all fossils ; there are no 

 recent shells which can be supposed to belong to the same 

 family ; and the condition in which they usually occur has in- 

 volved them in greater obscurity.* The characters which 

 determine their position amongst the ordinary bivalves are the 

 following : — 



1. The shell is composed of three distinct layers. 



2. They are essentially unsymmetrical, and right-and-left 

 valved. 



3. The sculpturing of the valves is dissimilar. 



4. There is evidence of a large internal ligament. 



5. The hinge-teeth are developed front the free valve, 



6. The muscular impressions are 2 only. 



7. There is a distinct pallial line. 



The oute«i' layer of shell in the EadioIIfce consists of prismatic 

 cellular structure (Fig. 232) ; the prisms are perpendicular to 

 the shell-laminse, and often minutely subdivided. The celLsi 

 appear to have been empty, like those of Ostrea (p. 407). f Thd 

 inner layer, which forms the hinge and lines the umbones is 

 *ub-nacreous, and very rarely preseiTed. It is usually replaced 

 by calcareous spar (Fig. 239), sometimes by mud or chalk, and 

 very often it is only indicated by a vacuity between the outer 

 shell and the internal mould (Fig. 244). The inner shell-layer 



* 1. Buch regarded them as Corals. 1840, Leonh. and Broan >ahrb. p. 573. 



2. Desmoulins, as a combination of the Tunicary and Ses.3i]e Cirripade. 



3. Dr. Carpenter, as a "group intermediate between the Conchifera a'id Cirripeda." 

 An. Nat. Hist. Xn. 390. 



4. Prof. Steenstrup, of Cqpenhagen, as Annelida. 



5. Mr. D. Sharpe refers Hippurites to the BaUni; Caprinella to the Chamaoea^. 



6. La PejTOUse considered the Hippurites Qrtkocercta ; the Radic'jtee^ O^rncta 



7. Goldfuss and D'Orbigny place them both with the .H^achiopoda. 



8. Lamarck and Rang, between the Brachiopoda and Ostracece. 



9. Cuvier and Owen, with the Lameliibranchiate bivalves». 



10. Deshayes, in the same group with ./Etheria. 



11. Quenstedt, between the Chamaceee and Cardiacea. 



t This is very conspicuous in Radiolites from the chalk ; a formation in which other 

 prismatic-cellular fossils are soUd. 



u 3 



