Weller — Kinderhook Faunal Studies. 451 



allel to the hinge-line; ligament external, attached to a 

 narrow, elongate, flattened area which is longitudinally 

 striate. 



The dimensions of three specimens are: length, 16, 13 

 and 11.3 mm.; width, 8, 7.5 and 6 mm.; length of hinge- 

 line, 11, 7.5 and 8 mm.; convexity, 4.5, 3.5 and 3 mm. 



Remarks. This is one of the common species in the Glen 

 Park fauna. It is closely allied to M. hamiltoniae from the 

 New York Hamilton fauna, but seems to be sufficiently dis- 

 tinct bv reason of the much finer radiating markinors of the 

 shell, which can only be detected with a magnifying glass 

 upon the best preserved specimens, while upon the Hamilton 

 shells they appear to be much more conspicuous, judging 

 from the illustrations alone. The Glen Park shells also 

 differ from M. hamiltoniae in the flattened area or broad 

 shallow sinus which extends obliquely across the ventral slope 

 from the beak to the ventral margin; the importance of this 

 sinus in the surface contour of the shells varies considerably, 

 but it is rarely absent altogether and is sometimes a very con- 

 spicuous feature; in M. hamiltoniae, on the other hand, it is 

 usually absent altogether, and in the illustrations of that 

 species is only shown on the larger individuals. The differ- 

 ence in size is also a distinction between the two species, the 

 Glen Park specimens never attaining the large size of M. 

 hamiltoniae. 



Another species with which M. aulcatus may be compared 

 is Elymella missouriensis M. & G., from the Chouteau lime- 

 stone of Central Missouri. The type of this species has not 

 been available for examination but a specimen so identified 

 from the Northview sandstone of Webster County, Missouri,* 

 has much more regular and stronger concentric markings. 

 A further cleaning of this specimen, however, has exhibited 

 a long posterior tooth parallel with the hinge line, and the 

 species should doubtless be referred to the genus Macrodon, 

 not far removed from M. hamiltoniae and M. sulcatus. An- 

 other specimen in the collections of Walker Museum, labelled 

 Elymella missouriensis from Chouteau Springs, Missouri, has 



* Traus. St. LoaLs Acad. Sci., vol. 9, p. 31, pi. 3, fig. 9. 



