CONCHIFRRA: 44} 
Geolcgical Socicty of London. In ait 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 valve of the Hippurite is not 
really very different from the upper valve in structure; the 
lamine are corrugated, leaving irregular porss, or tubes, parallel 
with the long axis of the shell, and often visible on the rim. 
The umbo of the upper valve of the Radiolite is marginal in the 
young shell. (Q.d. Geol. Soc., vol. x1. 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 igament. 
5. 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 Radiolite consists of prismatic 
cellular structure (Fig. 232); the prisms are perpendicular to 
the shell-laminz, and often minutely subdivided. The cells 
appear to have been empty, like those of Ostrea (p. 407).t Tine 
inner layer, which forms the hinge and lines the nmbones is 
sub-nacreous, and very rarely preserved. 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). ‘Phe inner shell-layer 
* 1; Buch regarded them as Corals. 1840, Leonh. and Broan vahrb. p. 575. 
2. Desmoulins, as a combination of the Tunicary and Sessile Cirr:pede. 
3. Dr, Carpenter, as a ‘‘ group intermediate between the Conchifera aud Cirripeda.” 
An. Nat. Hist. XII. 390. 
. Prof. Steenstrup, of Copenhagen, as Annelids, 
. Mr. D. Sharpe refers Hippurites to the Balani; Caprinella to the Chamacee. 
. La Peyrouse considered the Hippurites Orthocerata; the Radiclites, Ostracea. 
. Goldfuss and D’Orbigny place them both with the Brachwwpoda, 
. Lamarck and Rang, between the Brachiopoda and Ostracee. 
. Cuvier and Owen, with the Lameliibranchiate bivalves. 
10. Deshayes, in the same group with _dtheria. 
11. Quenstedt, between the Chamacee and Cardiacee. 
¢ This is very conspicuous in Radiolites from the chalk ; a formation in which other 
prismatic-cellular fossils are solid. 
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