100 NEW YORK STATE MUSEUM 



extremities are both subangular, the former obtusely, the latter 

 acutely so ; the anterior extremity is abruptly rounded. Beak small, 

 placed about midlength between the anterior and posterior extremi- 

 ties, umbo not very prominent, shell culminating a little above the 

 center ; umbonal ridge broad and traceable to the postventral extrem- 

 ity. Anterior, ventral and posterior slopes evenly rounded, cardinal 

 slope slightly concave. " Clavicular >J1 impression closely in front 

 of the beak, broad but not strong, nearly straight, extending obliquely 

 forward and downward about one-third the distance to the antero- 

 ventral margin. Near the post-dorsal angle are seen (see plate 5, 

 figure 13) small oblique hinge denticles or ribs that are found among 

 Ordovicic forms only in Clidophorus. Muscular impressions not 

 observed. Surface with several coarse varices of growth and fine, 

 very regular concentric lines which are barely visible with the naked 

 eye. 



Horizon and locality. Not uncommon in the dark gray to olive 

 colored sandy shale of the Snake Hill beds at Snake hill, Saratoga 

 county, N. Y. 



Remarks. This species is readily distinguished from 

 C. planulatus (Conrad) by the much greater convexity of 

 the shells. Doctor Ulrich writes me regarding the same that it 

 shows the beak nearer midlength than it is in any described species ; 

 that it is most like an undescribed type from the Southgate division 

 of the Eden at Cincinnati, and that it may be the same as one in the 

 basal part of the Martinsburg shale in Pennsylvania, the specimens 

 of which however are not good enough to decide the matter. The 

 general form is as in Whiteavesia but the characteristic Clidophorus 

 clavicle and the small oblique hinge denticles are present. 



Ctenodonta levata (Hall) 

 PI. 6, fig. i 



We have cited under this name certain small Ctenodontas from 

 the sandy shale of the Snake Hill beds at Snake hill and else- 

 where. Of the specimen figured Doctor Ulrich writes us that he 



1 This impression is currently termed the clavicular impression, but it is 

 obvious that it does not result from the prop or buttress, called clavicle in 

 more recent forms, which serves to reinforce the attachment of the 

 resilium or lower ligament. The "clavicle" of Clidophorus is a buttress or 

 lamina which by its position is clearly intended to support the anterior 

 adductor. Its relation to that muscle is well shown in our plate 5, figure IS- 

 The more recent Cucullaea, to cite an analogous case, possesses a like radial 

 buttress for the support of the posterior adductor. 



