SYNCYCLONEMA. 117 



each side, hut the species here under consideration (E. aviculatum, Swallow, sp.) 

 seems to have been gaping- on the sides above the middle. This being the case, I 

 am not sure the group is more than sub-generically distinct from Pseudamusium, 

 Brug., 1781*. It differs from Amusium mainly in having no internal costaa, and in 

 having the valves more nearly equal, with sometimes minute radiating stria?, and no 

 traces of a sinus under the anterior ear in either valve." There is, therefore, a 

 very close relationship between Entolium and Amusiam. 



The genus Pemopecten, Winchell, is closely allied to Entolium, and differs only 

 in having a crenulated hinge-line formed by a row of pits placed on either side of 

 the cartilage pit. I doubt if the genera could be distinguished if the hinge were 

 not exposed, as the external characters are so very similar. The two genera stand 

 to each other in the same relation that Aviculopecten stands to Euchondria, Meek, 

 and Crenvpecten, Hall, probably synonyms. This tendency of different genera to 

 develop along parallel lines is of great phylogenetic interest. Hall in 1885 (' Pal. 

 New York,' vol. v, part 1 ; ' Lamell.,' part ii, p. 57) gives a long and comprehensive 

 note on the synonymy of Entolium, and Mr. R. Etheridge, jun. (' Geol. Mag.' dec. ii, 

 vol. iv, 1877, p. 241), has also discussed the peculiar characters of the genus at 

 length, giving good and accurate drawings of the hinge-plate and showing the 

 generic identity of the Carboniferous and Jurassic species. Unfortunately no one 

 recognised that the genus occurred also in the Cretaceous beds, and had been 

 described by Meek in 18G4 (loc. cit.) as Syncyclonema, and therefore this generic 

 term has the priority to Entolium. Nor was it recognised that shells with a ribbed 

 internal surface, and other characters common to Amusium, existed in Carboniferous 

 beds side by side with those having a smooth internal surface. 



It is interesting, therefore, to note the persistence of Syncyclonema from 

 Carboniferous to Cretaceous times, and Amusium from Carboniferous to Recent. 

 It is most difficult to separate species of the two genera unless the interior is 

 exposed, and they are most closely related. 



At present only two species of Syncyclonema arc known from Carboniferous 

 rocks, one of these occurring in the Coal Measures ; but three species of Amusium 

 have been found. De Koninck referred five species from the Carboniferous 

 rocks of Belgium to Entolium, and at least two of these must be now removed to 

 Amusium. 



I have not been able to satisfy myself as to the condition of the ears in both 

 valves, i. e. whether both valves have the ears raised above the hinge-line so as to 

 prevent opening. The left valve always seems to have one or both ears so raised, 

 but the right valve has the hinge-line straight in the majority of cases. This may 

 be due to fracture, for there seems no necessity lor the valves to open if the sides 

 are not closed, and there may be compensation in the flange-like expansions of the 

 sides of the valve. 



