No. 9. — New Plagiostomia and Chismopnea. By Samuel 



Garman. 



Plagiostomia. 



In the Myliobatidae there are four well-marked genera. Three of 

 these have been established in some manner ever since the time of 

 Cuvier. The fourth, Aetomylaeus, species of which have been recog- 

 nized quite as long, has been lost in one of the others, hidden by resem- 

 blances. Outwardly its species are so like those of Myliobatis that they 

 have readily passed as congeneric. It was only upon the disclosure of 

 internal differences of the structure that the value of certain external pe- 

 culiarities was given proper consideration. The absence of a serrated 

 spine behind the dorsal fin, if not the result of accident, for one item, 

 has been looked upon as questionably sufficient for specific distinction. 

 On dissection of some of the species, however, this feature is found to be 

 associated with a division of each pectoral at the side of the head, that is, 

 with absence of pectoral rays connecting the cephalic portions with the 

 main sections of the pectoral fins, a characteristic of Aetobatus and not 

 of Myliobatis in which the species have been located heretofore. We 

 may note slight differences in the appearance of the pectorals opposite the 

 angles of the mouth after discovery of the lack of pectoral rays in these 

 positions, but the import of these features has been overshadowed by the 

 fact that in the species under notice they are associated with a dentition 

 practically the same as that of Myliobatis. Further comparisons assure 

 us that in these species we are dealing with a genus distinct from Mylio- 

 batis and considerably more specialized, as is evident from the division of 

 the pectorals and the loss of the serrated dorsal spiue. In brief summa- 

 tion, the new genus agrees with Myliobatis in dentition and in nasal 

 valves, while it differs in the divided pectorals and in the lack of a dorsal 

 spine ; and it agrees with Aetobatus in the pectoral divisions, while dif- 

 fering in regard to dentition, nasal valves, and absence of the spine. 

 These peculiarities, with others of less value perhaps, suffice to fix the 

 place of the new genus, Aetomylaeus, as intermediate between Mylio- 

 batis and Aetobatus. How the divergences and the accompanying af- 



