CONCHIFEEA. 441 



Geoiogical Societj^ of London. In all the genera the shell con- 

 sists of three 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 yalve of the Hippurite is not 

 really very 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 RadioJite 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 

 yalved. 



3. The sculpturing of the valves is dissimilar. 



4. There is evidence of a large internal ligament. 



o. The hinge-teeth are developed from the free valve. 



6. The muscular impressions are 2 only. 



7. There is a distinct pallial line. 



The outer layer of shell in the Eadiolite consists of prismatic 

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

 the shell-laminae, and often minutely subdivided. The cells 

 appear to have been empty, like those of Ostrea (p. 407). t The 

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

 sub-nacreous, and very rarely preserved. It is usually replaced 

 by calcareous spar (Fig. 239), sometimes bj' 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, Lconh. and Bronn Jahrb. p. 573. 

 2. Desmouliiis, as a combination of the Tunicary and Sessile Cin-ipede. 

 :^. Dr. Carpenter, as a " group intermediate between the Conchifcra and CirripedaJ''' 

 An. Nat. Hist. Xll. 390. 

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



6. Mr. D. Sharpe refers Hippuritea to the Balani; Caprinclla to the Chamacere. 

 6. La PejTouse considered the Hippurites Orthoceratu; the Eadiolites, Oatracea, 

 7- Goldfu^s and D'Orbigny place tliem both with the Brachiopoda, 



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



9. Cuvier and Owen, with tlie Lamellibranchiate bivalves. 



10. Deshayes, in the same group with JEtheria. 



11. Quenstedt, between the Ckumacece and Cardiacefe.' 



t This is very conspicuous in Eadiolites from the chalk ; a formation in which otlier 

 prismatic-cellular fossils are sohd. 



u 3 



