44 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



to have been figured or described; but Goldfuss and D'Orbigny 

 have figured a mould of its interior, and I obtained a similar prepa- 

 ration (fig. 7) by removing the upper valve from one of Mr. Pratt's 

 specimens with a hammer and chisel. 



Figs. 6 & 7. — Upper valve ofH. Toucasianus. 



Fig. 7. 



Fig. 6. Exterior of the upper valve ; \. 



Fig. 7. Mould of the interior of upper valve ; f . I, m, n, duplicatures ; x, fracture, 



showing canals ; c, cartilage ; u, left umbo ; — the arrows indicate the 



supposed direction of the branchial currents. 



This mould shows the umbo turned forwards, and having a deep 

 furrow on each side, caused by processes from the upper valve. On 

 the dorsal side of the umbo, close to the ligamental inflection, is a 

 small conical elevation (omitted in the figures of Goldfuss and 

 D'Orbigny) representing the cartilage or one of its divisions. 



A plaster-cast taken from this mould gives the form of the interior, 

 to a certain extent ; that is to say, it shows the umbonal cavity, the 

 cartilage-pit, a deep furrow winding round the adductor and siphonal 

 inflections, and the bases of the hinge-teeth. 



With the help of this mould I filled up the umbonal cavity of the 

 other specimen (the lower valve of ^. radiosus), and then took from 

 it a plaster- cast (fig. 5), which gives what I believe to have been the 

 form of the upper valve with its processes complete. 



To test the correctness of this model, I made a number of sections, 

 both transverse and longitudinal, of Hippurites in which both valves 

 were preserved. These show that the two prominent hinge-teeth 

 were extensively under-cut by the umbonal cavity (fig. 8, u), so as 

 to appear suspended by thin plates. Each tooth supports a process 

 corresponding in shape to the muscular impressions in the lower 

 valve ; the anterior projecting horizontally ; the posterior vertical 

 and tooth-like, longer indeed than the tooth to which it is attached ; 

 but thinner than in the model, not nearly filling the cavity for its re- 

 ception (fig. 3, «'). Both these muscular apophyses are under-cut, so 

 as not to interfere with the channel which winds round the inflections. 



This explanation of the hinge-teeth and muscular processes has 



