EUMICROTIS. 43 



consider J. de Carle Sowerby's species A. gentilis to be good, but have doubts as to 

 the value of P. scalaris of this author. 



M'Coy described as new seventy-five species of Pectiuiform shells in his earlier 

 work, and sixteen of these were founded on right valves. Many of his specific 

 names are synonyms, representing opposite valves or different stages of growth of 

 the same shell. Several of the types are too poor to recognise their true affinity, and 

 several others had been previously described by other writers, while two species, 

 Ariciila piilchelhi and Per ten flabellulum, do not belong to the family. Of these two 

 shells the first is a fragment of Pteronites persulcatus, and the second, \ think, the 

 interior of Athyris planosulcata. I am able to retain only about twenty species. 



In his second work, M'Coy described three other species, Aviculopecten docens, 

 A. planar adiatus, and A. Buthveni. The first is Portlock's Pecten semicostatus. 

 The second species, founded on a left valve, had been previously described by 

 M'Coy as P. tabulatus ; and the last species, retained for the present, seems to me 

 to be probably a damaged and full-grown specimen of A. interstitialis, Phillips. 

 Do Koninck described three species of Pecten and twelve species under the genus 

 Avicula in his early work, 1842 ; but in 1 885 he described eleven species of Butotia, 

 sixteen species of Strcblojrteria, forty-six species of Aviculopecten, five species of 

 Entolium, and six species of Limatulina, a very large number of which I regard as 

 synonyms, either representing different stages of growth or simple variations, not 

 of specific value. 



The study of the family of Pedens, as represented in Carboniferous beds, has 

 been very troublesome owing to the poor material and the difficulty of obtaining 

 specimens well enough preserved to study the important details. The surface- 

 ornament of shells belonging to different genera is often so similar that, unless the 

 hinge-line, and especially the ears, are present, a correct determination of the 

 affinities of the shell is almost impossible. In addition, the hinge-plate is rarely if 

 ever exposed even in the larger specimens. The poor and imperfect condition of 

 many of the types has also caused a difficulty; and to this and the non-recognition 

 of opposite valves of the same shell is due the inordinately large number of 

 supposed species described. 



Genus Eumickotis, Meek, 18G4. 



Eumicrotis, Meek, 1804. Amer. Journ. Sci., 2nd ser., vol. xxxvii, p. 218. 



Meek and Hoyden, 1865. Pal. Upper Missouri, p. 53 (Smithsonian 

 Contrib. Knowl., vol. xiv, No. 5). 

 Rutotia, de Koninck, 1885. Ann. Mus. Roy. d'Hist. Nat. Belg., torn, xi, p. 196. 



Generic Characters. — Shell suborbicular, slightly oblique, the left valve mode- 

 rately tumid, without ears. The right valve flattened, with a small anterior ear. 



7 



