272 THE LAMELLIBRANCHIA 
operculiform ; fossil from the Cretaceous (Fig. 244, B). Matheronia, 
Munier-Chalmas ; fossil from the Cretaceous. Faminy 2. CaPRINIDAE, 
dOrbigny. 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 
—Cuprina, @Orbigny ; the free 
valve larger than the fixed and 
coiled (Fig. 244, C). Caprinula, 
dOrbigny ; fixed valve elongated 
and conical, free valve small and 
coiled (Fig. 244, D). Caprotina, 
d’Orbigny. Ichthyosarcolites, Des- 
marets. Plagioptychus, Matheron. 
Polyconites, Roulland, Famtny 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). 
Valletia, Munier-Chalmas. 
Baylea, Munier-Chalmas. The 
4, Si gone of fool, Chamaces and Eudlstes, two following \ families, desig: 
Caprina adverst ; D, Caprinula, Baylei ; E, Mono- nated by the common name of 
rete ec F, Radiolites angeiodes. (Chiefly Rudistae, aie 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. Famity 4. 
RaADIOLITIDAE, Gray. Shell conical or biconvex, without canals in the 
external layer, Genera—RAadiolites, Lamarck ; valves ornamented with 
longitudinal costae ; a ligament present ; from the Cretaceous (Fig. 244, F). 
Biradiolites, VOrbigny ; no ‘ligament; Cretaceous. Faminy 5. Hr1p- 
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 
Fic. 244. 
