EASTMAN : ST11UCTUKE AND RELATIONS OF MYLOSTOMA. 23 



for reciprocal marks of wear ; (3) the reconstruction here shown is in 

 harmony with ontogenetic evidence ; and (4) the same disposition has 

 been found to hold true also for Dinomylostoma (vide infra). 



It may be noted further that there does not appear to be any certain 

 trace of the median ventral plates, and in Dean's restoration of the 

 ventral armor one perceives that the antero- and postero-ventro-laterals 

 have been interchanged. The restoration of the headshield and con- 

 figuration of the dorso-median plate, as shown in Dean's figure 3 

 must be understood as largely conjectural ; and his statement that the 

 orbits are "placed dorsally and somewhat closely together, characters 

 which perhaps might be expected in a fish of ray-like habits," cannot 

 be confirmed. Mylostoma agrees with Coccosteus and Dinichthys, rather 

 than with Homosteus, as regards position of the orbits, and there is no 

 indication that the body was depressed dorso-ventrally in any of these 

 genera. 



In Plates 1 and 2 are shown a number of palato-pterygoid plates 

 belonging to Mylostoma variabiles some of which are type specimens ; and 

 in Plate 3 are represented two of the mandibular dental plates belonging 

 to the same species (Figs. 19, 20), and also the type specimen of M. terrelli 

 Newberry (Fig. 21). None of the originals selected for illustration 

 seem to require further comment than will be found in the explanation 

 of the plates. For having generously placed material belonging to the 

 American Museum of Natural History at the writer's disposal, his appre- 

 ciative thanks are due and here rendered to his friend Dr. Dean, honor- 

 ary curator of fossil fishes, as well as to his assistant, Mr. L. Hussakof. 



Dinomylostoma Eastman. 

 Arthrodiran fishes exhibiting characters transitional between Mylostoma and 

 Dinichthys. Trltoral palato-pterygoid dental plates adapted for crushing, but the 

 opposing lower dentition very similar to that of Dinichthys, except that the 

 functional margin is thickened into a broad, more or less smooth, and regularly 

 concave grinding surface, contracting in front and elevated into a blunt symphys- 

 ial beak. Vomerine teeth subtrihedral, slightly prehensile, and of a general 

 Dinichthys-like aspect. Dorso-median with prominent inferior carina terminating 

 in an excavated posterior process ; other plates of the abdominal armor resembling 

 those of typical Coccosteans, the external surface covered with fine vermiculating 

 rugas. 



Dinomylostoma beecheri Eastman. 

 Plate 1, Figs. 4, 5 ; Plate 2, Figs. 13, 14, 16, 17 ; Plates 4, 5. 

 Amer. Journ. Sci. 1906, ser. 4, 21, p. 137, Text-Fig. 2. 



The specific characters of this form are included under the foregoing 

 generic diagnosis. That which is regarded as the most distinctive 



