88 Mr. W. King on certain Genera 



eighth of an inch of the inner surface of the dorsal valve, 

 leaving thereby just sufficient space for the thickness of the 

 animal's mantle. I am not aware that any opinion has been 

 hazarded on the use of this singular process ; there is every rea- 

 son to believe however, from the remarkable modifications which 

 the cardinal muscular fulcrum occasionally undergoes, that both 

 are strictly homologous. In some fossil Terebratulas the cardinal 

 muscular support is erect and unusually elongated, particularly 

 in a cretaceous species, probably T. pectiniformis ; it appears to 

 be the same in Orthis eximia, Vern. ; and in the existing Tere- 

 bratula rosea it is very much lengthened, but situated on an 

 elevation rising out of an excessively dilated cardinal plate. 



In the hinge of the same valve are situated two depressions 

 or sockets for the condyles of the dorsal valve, one on each 

 side of the cardinal muscular support : the socket-walls are very 

 much expanded laterally, so as to form two prominent plates, 

 which descend, curving in towards each other at the same time, 

 to a little below the origin of the cardinal muscular support, 

 where they nearly touch a slightly elevated vertical plate, which 

 stretches to about half-way along the medio-longitudinal line 

 of the valve. Their origin and position, and the peculiarity 

 next to be described, are highly in favour of these plates consti- 

 tuting a divided crural base*. Each of the crural plates, on its 

 lower part, gives off a slender lamelliform process, which curves 

 (the concave side upwards) towards the anterior end of the me- 

 sial plate of the dorsal valve, but a little to one side of it ; the 

 process now makes a sudden bend upon itself, curves downwards 

 and postero-laterally, till it nearly touches the end of the car- 

 dinal line ; here it makes a sharp forward curve, runs along the 

 side, and afterwards along the front of the valves, at the distance 

 of a quarter of an inch from their margin, to nearly the medio- 

 longitudinal line of the shell ; further I have not been able to 

 trace it. This is the course of both processes : they thus form 

 two symmetrical subgyrate appendages, which remind one of the 

 spiral coils of the Spirifers and the folded loop of the Terebra- 

 tulas. It is to be hoped that sufficient has been adduced to 

 show the generic difference between Strigocephalus and Penta- 

 merus, which has occasionally been doubtedf. The difference 

 is such as to induce me to place the former in the family Spiri- 



* This view is further supported by the striking resemblance which these 

 plates bear to the concave crural base of Martinia {Terebratula) kyalina, 

 Buch. This species is interesting in another point of view, as from its 

 external resemblance to Strigocephalus, we are warranted in supposing that 

 both are intimately related to each other, although belonging to separate 

 genera. 



f " The difference between Strigocephalus and Pentamerus appears to 

 me not very important." — Phillips, Palaeozoic Fossils of Cornwall, &c, p. 55. 



