42 C'ATALOG OF F9SS1L FISHES IN THE MUSEUM 



Plate 7, fig. I, so that no lengthy description of it is necessary. Its 

 outstanding features are the low, broad beak, and the position of the 

 secondary cusp, which is situated far back, near the middle of the 

 functional margin. It is this feature which has enabled us to identify 

 as of this species one or two upper shear teeth, since the groove worn 

 in the sheer plate by the secondary mandibular cusp must be situ- 

 ated farther back in this species than in others. 



Antero-superognathal. — The left element is represented; it is badly 

 fractured, the median lateral flange being broken away. The main 

 cusp and the articulating process are however present. 



Postero-superognathal. — The left postero-superognathal lay beneath 

 the median occipital, and a complete cast of it was obtained. It is 

 shown in the restoration, Plate i. There are no posterior denticles 

 and the inner face exhibits a deep groove worn by the posterior 

 mandibular cusp. 



Dorsomedian. — Only a portion of this plate is preserved. A cast re- 

 constructed from it measures 1 5 inches in length, and 1 7 inches in great- 

 est width. The surface of the plate is finely tuberculated, the tubercles 

 being, if an5^hing, smaller than those of D. pustulosus. It is also note- 

 worthy that the plate is strongly arched from side to side, and, like 

 the head shield, apparently suffered little deformation, owing to its 

 favorable preservation in a concretion. 



Lateral or ^'Clavicular.'" — This element was represented in the 

 opened concretion as an impression, so that a cast could be taken of 

 almost the entire plate. Its form and proportions are clearly indi- 

 cated in Plate 7, figure 2. The posterior margin measures 25.4 cm. 

 in length, and the inwardly directed arm, 12.6 cm. The outer surface 

 is partially tuberculated. 



Ventral Armor. — Of the ventral armor there are preserved the pos- 

 tero-ventromedian, an antero-ventrolateral and both postero-ventro- 

 laterals. 



The postero-ventromedian (PL 22, fig. i) is shown in the specimen in 

 inner, or visceral view. It is 25.4 cm. in length, and 20.3 cm. in great- 

 est width. It is much wider in proportion to its length than its homolog 

 in D. terrelli, and as the anterior end is missing, it cannot be stated 

 whether it was fused with the anterior element into a single ventro- 

 median, as in D. terrelli, or was united with it by a socket joint as in 

 most other species. 



The antero-ventrolateral preserved is that of the left side, shown in 

 inner or visceral view. Its inner margin is broken away. The plate 



