398 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 24, 



a distinct family, related most nearly to the Chamidce and the Car- 

 diadce. 



The Hippurite was shown to have its valves articulated by a hinge 

 essentially like that of the Radiolite. The internal ligament or 

 cartilage is divided into two portions, situated at the sides of the 

 first or most anterior of the three inflections of the outer shell-wall ; 

 this ligamental inflection is very prominent in the Hippurites corrm- 

 vaccinum, but obsolete in H. bioculatus. Two prominent straight 

 teeth are developed from the upper valve, parallel vsith the hinge- 

 margin, in H. bioculatus and H. radiosus, transverse to it in H. cornu- 

 vaccinum. The anterior tooth supports a curved horizontal muscular 

 apophysis, shaped like the adductor impression in the lower valve ; 

 the posterior tooth has a long vertical process, sometimes more pro- 

 minent than itself, which is received by the cavity between the first 

 (or hgamental) and second (muscular) inflection, which corresponds 

 to the muscular ridge in Dicer as and Cardilia. The third ridge in 

 the lower valve of the Hippurite, which meets a corresponding in- 

 flection of the upper valve, was compared to the siphonal ridge of 

 certain bivalves, which divides the inhalent from the exhalent current 

 of water. 



In a series of specimens of Radiolites Hoeninghausii, presented by 

 Mr. Pratt to the British Museum, there was evidence that the umbo 

 of the upper valve was marginal when young ; and the ligamental 

 inflection, though obsolete externally, was always manifest inside the 

 upper valves which had lost their inner shell-layer. 



The Radiolites Mortoni (Mantell) was shown, by specimens from 

 the chalk of Kent, exhibited by Mr. M. Wright, to have possessed 

 a thin internal layer vdth grooves, rather than sockets, for the teeth 

 and muscular process close to the side, there being no projecting 

 ligamental plate ; the interior (of the lower valve) was divided into 

 water-chambers by thin concave plates, as in the various foreign 

 species. 



The difference between the shell-structure of the upper valves of 

 Hippurites and Radiolites was compared to the difference between 

 Rhynclionella and Terebratula, and held to be, in this case, only of 

 generic importance. 



The genus Caprotina, D'Orbigny, was shown to include certain 

 species (e. g. C. 4-partita) which could be compared to Radiolites, 

 the hinge-teeth supporting plates for the attachment of the shell- 

 muscles ; the anterior tooth being fiirther connected with a plate 

 which divides the umbonal cavity of the upper valve in two. The 

 first and fourth lobes of Cap)rotina were held to represent the internal 

 cartilage ("accessory apparatus") of Radiolites. 



Evidence was adduced to show that the fixed (or dorsal) valve of 

 Caprinella and Cajrrinula was always turned away from the spiral 

 valve, with more or less of a sigmoid flexure, and not as in the 

 restoration given by M. d'Orbigny. 



The genera Reqidenia (R. Loasdalei) and Monoplenra (J/, im- 

 hricata) were considered to be more nearly related to Diceras and 

 C7ia»ia. 



