VERTEBRATES. 25 



Most of the specimens contained in the collection, which have been grouped 

 together under the above name, exhibit, in the flattening of the central portion 

 of the anterior and posterior surfaces, a strongly marked character by which 

 they may be readily distinguished from any of the species now or heretofore 

 described. There are other specimens, however, in which this character is not 

 so distinctly visible, and which are not unlike G micropus in the form of the 

 median cone, but have a larger base. The teeth of G turritus have also a simi- 

 larly striated surface, and an approach to the angular outline of those under 

 consideration, but in that species the lateral denticles are much larger, and the 

 base is relatively thinner and smaller. For the present, therefore, it seems 

 necessary to regard these teeth as specifically distinct from those with which 

 we have compared them, but it is by no means certain, that by the discovery 

 of new material, it will not be shown hereafter that the teeth now described 

 under the names of G micropus, G. robustus, C. angulatus, and G. turritus, form 

 portions of the dentition of the same fish. 



Figure 7, posterior face and section of median cone ; 8 and 8 a, anterior face 

 and base of another specimen. 



Formation and hculity: Keokuk limestone, Hamilton and "Warsaw, Illinois. 



Cladodus zygopus, N. and "W. 



PI. 1, Fig. 9, 9 a, and 10. 



Teeth small, as broad as high; base narrow, with a rela- 

 tively deep sinus in front, which gives it somewhat the form of a 

 yoke, and has suggested the name chosen ; median cone conical, 

 broad at base, rapidly narrowed to an acute summit, distinctly 

 and evenly striated throughout, curved backward, anterior face 

 flattened and deeply excavated below by the basal sinus ; sides 

 sharply angled below at the junction of anterior and posterior 

 surfaces, ancipital towards the point, posterior face rounded ; 

 lateral denticles, two on each side, sub-equal ; flattened before, 

 rounded behind, striated, acute. 



This highly ornamented and pretty species belongs to a group which includes 

 C. Hibberti, Ag., G. gracilis and G. costatus, Nob. All of these are small in 

 size, have two lateral denticles on either side ; have a distinct sinus at the base 

 of the median cone in front; have the median cone flattened in front, rounded 

 posteriorly with cutting edges, and marked with distinct, sharp, longitudinal 



—4 



