272 



THE LAMELL1BRANCH1A 



operculiform ; fossil from the Cretaceous (Fig. 244, B). Matheronia, 

 Munier-Chalmas ; fossil from the Cretaceous. FAMILY 2. CAPRIXIDAE, 

 d'Orbigny. Shell inequivalve ; the fixed valve spiral or conical ; the 

 free valve not operculiform but coiled or spiral ; ligament internal ; sub- 

 stance of the shell generally 

 pierced with large parallel canals. 

 An exclusively fossil family, 

 from the Cretaceous. Genera 

 Caprina, d'Orbigny ; the free 

 valve larger than the fixed and 

 coiled (Fig. 244, C). Caprinula, 

 d'Orbigny ; fixed valve elongated 

 and conical, free valve small and 

 coiled (Fig. 244, D). Caprotina, 

 d'Orbigny. Ichthyosarcolites, Des- 

 marets. Playioptychus, Matheron. 

 Polycmiite*, Roulland. FAMILY 3. 

 MONOPLEURIDAE, Munier- 

 Chalmas. Shell very inequi- 

 valve ; the fixed valve conical or 

 spiral, the free valve operculiform 

 and slightly or not at all spiral ; 

 ligament external. No canals in 

 the substance of the shell. An 

 exclusively fossil family, from 

 the Cretaceous. Genera Mono- 

 pleura, Matheron (Fig. 244, E). 

 Valletta, Munier-Chalmas. 

 Baylea, Munier-Chalmas. The 

 Some genera of fossil Chaiuacea and Rudistae. t followin" families dpsif 



A, Diceras arietinum; B, Bequienia ammonia; C, l le ' ( 



Caprina adversa; D, Caprinula Baylei; E, Mono- nated by the common name of 

 pleura imbricata ; F, Radiolites angeiodes. (Chiefly r> j ,. i i n-iiii. 



after d'Orbigny.) Mtmutae, are closely allied to the 



preceding : they also comprise 



some extinct marine forms from Secondary deposits. These animals, of 

 littoral and often gregarious habit, were fixed by the conical and more or 

 less elongated right valve ; the adductor muscles were not inserted perpen- 

 dicularly to the surface of separation of the two valves ; the free left 

 valve has a sub-central umbo, is not spiral, and is furnished with promi- 

 nent myophorous apophyses to whose external faces the muscles were 

 attached ; this valve was only movable in a vertical direction. FAMILY 4. 

 RADIOLITIDAE, Gray. Shell conical or biconvex, without canals in the 

 external layer. Genera Radiolites, Lamarck ; valves ornamented with 

 longitudinal costae ; a ligament present ; from the Cretaceous (Fig. 244, F). 

 Biradiolites, d'Orbigny ; no ligament ; Cretaceous. FAMILY 5. HIP- 

 PURITIDAE, Gray. Fixed valve long, cylindro- conical, with three 

 longitudinal furrows, corresponding internally to two pillars which serve 

 . to support the siphons. Anterior adductor muscle with two separate 

 insertions on the fixed valve. Genera Hippurites, Lamarck ; Cretaceous. 

 Arnaudia, Bayle ; Cretaceous. The family Diceratidae, the most ancient 



FIG. 244. 



