WOODWARD HIPPURITIDiE. 



43 



and empty, like the cellular structure between the laminae of certain 

 Oysters. 



Figs. 4 & 5. — Upper and lower valves q/" Hippurites radiosus, Desm. 



Fig. 4. 



Fig. 5. 



Fig. 

 Fig. 



4. Interior of lower valve (^). 



5. Model of upper valve : a, a', adductor impressions and processes ; c, c, car- 



tilage-pits ; t, f, teeth and dental sockets ; u, umbonal cavity; p, orifices 

 of canals ; I, ligamental inflection ; m, muscular, n, siphonal inflections. 



The umbonal cavity of the lower valve is contracted by three 

 ridges, produced by inflections of the outer wall ; they correspond to 

 the three furrows on the outside. The first, or ligamental, inflec- 

 tion is very slight, and opposite to the centre of the hinge, which 

 consists of two deep dental sockets, divided by a tooth-like process, 

 and separated from the shell-wall by two narrow and deep pits for 

 the internal ligament {cartilage) . In front of the hinge is a large 

 muscular impression consisting of two portions, and answering to the 

 anterior adductor, which usually consists of two elements in ordinary 

 bivalves. Behind the hinge, and between it and the second inflec- 

 tion, is a deep pit, marked inside with the impression of the posterior 

 adductor muscle. This inflection, therefore, appears to represent 

 the lamina which supports the posterior adductor in the fossil genus 

 Dicer as, and in the recent Cardilia. 



The third inflection may possibly correspond to the ridge which 

 indicates the division of the siphonal orifices in some bivalves, such 

 as the LedcE and Trigonia aliformis. 



In H. cornu-vaccinum the form of the interior is rather diff'erent 

 (fig. 3) *. See also PI. IV. figs. 2 and 3. The ligamental inflection 

 is very deep, and the dental sockets are placed across the interior of 

 the shell, instead of parallel with the side, leaving a shallow cavity 

 in front, which may perhaps have lodged the internal ligament, for 

 there are no distinct ligamental pits in the lower valve of this species. 

 Hippurites which have lost their inner layer exhibit either two or three 

 longitudinal ridges, accordingly as they belong to the same division 

 with H. cornu-vaccinum, or with H. radiosus and bi-oculatus. 



The interior of the upper valve of the Hippurite appears not 



* Compare D'Orbigny's figure, Paleont. Fran^aise, Ter. Cret. pi. 527. f. 2. 



