OF DENISON UNIVERSITY. 63 



Myalina michiganensis, ^Vin ? 



(Cf. J/, siuallozu.) 



(Plate IV, Fig. 6.) 



The only difference between this species and M. swallovi of the 

 coal-measures is in the greater size of the protuberance under the beak, 

 but Meek says specifically: " This species (M. swallovi) varies some- 

 what in the size of the protuberance under the beaks ; in some indi- 

 viduals it is well developed, and gives the shell much the form of 

 Modiola; while in others it is smaller, so as to present more the ap- 

 pearance of a Mytilus:' Perhaps it may be sufficient to indicate the 

 probable genetic relations of these forms. 



Modiola luaverllensis, sp. n. 

 (Plate I, Fig. 9 ; Plate IV, Fig. lo; Plate VIL Fig. 29?) 



(There has been no opportunity to consult the description of M. 

 metella, H. so the possibility of agreement with this Chemung form 

 is not excluded.) 



Nearlv like M. nevadensis, Wal, but less elongate. Transverse- 

 ly elongate, narrowly and uniformly rounded anteriorly, lower outline 

 very gently curved, posterior end rather more broadly rounded than 

 the anterior, the outline above diverging rather strongly to a point be- 

 tween the middle and the posterior third, where it meets the nearly 

 straight hinge. Beak posterior, somewhat overhanging. Post umlx)- 

 nal slope quite well marked near the convex umbonal region, poste- 

 rior part of shell rather flat. Length somewhat greater than twice the 



width. 



A number of casts have been found in the freestone of the mid- 

 dle Waverly. Those specimens preserving the shell markings differ 

 slicrhtly from the cast first referred to and have fine concentric striae. 

 These may indicate a distinct species, but probably the apparent dif- 

 ferences correspond to the different states of preservation. 



