60 Dr. J. W. Gregory— The Age of the Morte Slates. 



Witboufc the opportunity for a careful study of a considerable 

 series of specimens in various conditions of distortion, the specific 

 determination of crushed specimens of Rhynchonella and Orthis is not 

 a task lightly to be undertaken. I was accordingly more interested 

 in the generic determination of the three remaining species. 



The species named Pterincea Mortensis is interesting as 

 containing the most recognizable fossils in the collection. Of the 

 two specimens figured, that illustrated by pi. xi, fig. 16 seems to 

 me the better. Its umbo is situated at ^Aj- of the length of the 

 hinge-line from the anterior end ; in Pterincea the umbo is much 

 more forward, occurring at about only half that distance from the 

 anterior end. Thus, taking the distance of the umbo from that point 

 at 1, then in six casually chosen species of Pterincea the length of the 

 hinge-line would be as follows: 4-1, 44, 5, 5-1, 54, and 6-1. In 

 Dr. Hicks' two species the length of the hinge-line would be 2-17 

 and 2-6. 



Moreover, the umbo of Dr. Hicks' specimens are higher than in 

 Pterincea, and the form of the shell is j^ectinoid, rather than having 

 the trigonal shape of Pterincea. All the known characters of Dr. 

 Hicks' new species seem to me to indicate that the fossils should be 

 removed from the family Pteriniidce, and included in the genus 

 Avieulopecten. Taking ten species of that genus at random, the 

 relation of the position of the umbo to the length of the hinge-lines 

 varies from 1-9 to 2-5; the figures are as follows: 1-9, 1-9. 1'94, 

 202, 2-15, 2-17, 2-18, 2-25, 2-3, and 25. The fact that Dr. Hicks' 

 specimens agree in this important character with Avieulopecten 

 rather than with Pterinaa, may be regarded as due to the distortion 

 of the specimens. This argument might be applied to the specimen 

 shown on pi. xi, fig. 17, but in the case of the other (fig. 16) the 

 fossil has only been somewhat lengthened ; and as the lengthening 

 has been uniform, this could not have altered the relative position 

 of the umbo on the hinge-line. The genus Pterincea occurs both in 

 the Silurian and Dev(mian systems ; but Avieulopecten is typically 

 Upper Palagozoic. 



In the next species, Modiol apsis Barricanensis, Hicks, the generic 

 determination is again the important point. The species is said to 

 approach most nearly to the Silurian Modiolopsis suhalata, Hall; but 

 the resemblance between the two species does not seem to me very 

 striking, and, in fact, I doubt if they belong to the same genus. 



There are two specimens figured as belonging to this species ; that 

 shown on fig. 15 is too young for determination by itself, but the 

 other specimen is more instructive. It is probably a member of the 

 family Modiolopsidge; but all doubt on this point cannot be removed 

 without further exposure of the anterior margin of the shell. 

 Assuming that it is one of the Modiolopsidee, it seems to me to belong 

 to Modiomorpha rather than to Modiolopsis. The distinction of 

 the two genera rests on the dentition, which cannot be seen in either 

 specimen ; the affinities of the species can therefore only be 

 provisionally determined by comparing its form with that of the 

 variations seen in the two genera. The larger specimen of Dr. Hicks' 



