280 MANUAL OF THE MOLLUSCA. 



strata, and abouad in many parts of the Peninsula, the Alps and E. Europe, 

 where the equivalent of the Lower Chalk has received the name of " Hippu- 

 rite limestone." They occur also in Tm-key and in Egypt, and Dr. F. 

 Roemer has found them in Texas and Guadaloupe. 



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 involved them in greater obscm-ity.* The 

 characters which determine their position amongst the ordinary bivalves are 

 the following: — 



1. The shell is composed of two 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 ft'om the free valve. * 



6. The muscular impressions are 2 only. * 



7. There is a distinct pallial line. 



The outer layer of shell in the Hippurite and Radiolite consists of pris- 

 matic cellular structm-e (fig. 123) ; the prisms are perpendicular to the shell- . 

 lamina?, and subdivided often minutely. The cells appear to have been | 

 empty, like those of Ostrea (p. 254). f The inner layer, which forms the \ 

 hinge and lines the umboues is sub-nacreous, and very rarely preserved. It i 

 is usually replaced by calcareous spar (fig. 200), sometimes by mud or chalk, j 

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

 the internal mould (fig. 205). The inner shell-layer is seldom compact, its 

 lamellae are extremely thin, and separated by intervals like the water-chambers | 

 of Spondylus ; similar spaces occur in the deposit, filling the umbonal cavity : 

 of the long-beaked oysters. | 



* J . Buch regarded them as Corals. 1840, Leonh. and Bronn Jahrb. p. 573. 



2. Desmoulins, as a combination of the Tunicary and Sessile Cirripede. | 



3. Dr. Carpenter, as a "group intermediate between the Conchifera and Cirri- 

 peda." An. Nat. Hist. XII. 390. i 



4. Prof. Steenstrup, of Copenhagen, as Anellides. j 



5. Mr. D. Sharpe refers Hippurites to the Balani; Caprinella to the Chamaceae. i 



6. Lapeirouse considered the Hippurites Orthocerata; the Radiolites, Ostracea. 



7. Goldfuss and D'Orbigny place them both with the Brachiopoda. 



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



9. Cuvier and Owen, with the Lamellibranchiate bivalves. I 



10. Deshayes, in the same group with JEtheria. \ 



11. Quenstedt, between the Chainacece and Cardiacece. 



t This is very conspicuous in Radiolites from the Chalk ; a formation in which i 

 other prismatic-cellular fossils are solid. ; 



I The water-chambers in some of the cylindrical Hippurites are large and regular, j 

 like those of the fossil corals Amplexus and Ctjathophyllum. A section of Hippurites ■] 

 bi-oculatus passing through only one of the dental sockets, resembles an Orthoceras . 

 with a lateral siphuncle; whilst a Caprinella (fig. 207J, which has lost its outer layer, ,; 

 might be mistaken for a sort of A mtnonite. | 



