174 CARBONIFEROUS LAMELLIBRANCHIATA. 



Mr. Btheridge lays stress upon the " byssal sinus extending from the beak as 

 a pronounced groove in each valve " (op. supra cit.) in his description of Avicula 

 Hendersoni. 



Parallelodon divisus is probably the oldest Carboniferous example of the Area 

 family ; and, therefore, the peculiar characteristics of the species are of importance 

 as indicating the line of descent of the genus. The contour of the posterior part 

 of the shell is distinctly aviculoid, as is also the deep byssal sinus, which, however, 

 in the species in question, is situated much further backwards than in any 

 member of the Aviculidas. It is of interest to note that the typical hinge-plate of 

 Parallelodon was already acquired, but that the radiating lines, which are such a 

 marked feature in the ornamentation of the surface of so many species, are 

 conspicuously absent. 



Mr. Kirkby places the horizon at which this species occurs on the Fifeshire 

 coast at from 3.000 to 3,800 feet below the Hurlet Limestone. The band of 

 limestone, fifteen inches thick, on the upper surface of which the species occurs, is 

 strewn with shells and debris, and was evidently an old shore. Fourteen species 

 of Gasteropoda and seven species of Lamellibranchs have been recognised in it with 

 Crustacea, fish, and plant remains by Mr. J. W. Kirkby. Perhaps the commonest 

 fossil is Schizodus Pentlandicus, Rhind, with which P. divisus is also associated at 

 Woodhall, Water of Leith ; many of the associated fossils also occurring in this 

 other locality. 



Lithodomus carbonarius, Hind (see p. 80), occurs both in the Randerstone and 

 Eskdale beds. 



The species described by M'Coy as Bijssoarca lanceolata and B. clathrata do not 

 appear in this work, either as species or synonyms. The type specimen of the 

 first is still extant, but I cannot definitely decide on its true generic affinities, and 

 as there are no other specimens at all like it, I have not described it under the 

 genus Parallelodon. The type of Byssoarca clathrata has disappeared, and I 

 think it probable that what was described as a new species was only a very well- 

 preserved juvenile form of one of the reticulate species. 



Cucullaea arguta, Phillips, retained by M'Coy and others, I do not consider to 

 belong to the genus Parallelodon. The type specimen is preserved in the 

 Gilbertson Collection of the British Museum (Natural History). 



Arcafaba, de Koninck, is catalogued from the West of Scotland by Armstrong 

 aud Young, but I have not been able to identify this species in any Scotch 

 collection. The specimens so named are probably the internal casts of P. Geinitzi. 



