PLATE 75. 



MOBULIDAE. 



Fig. 1-2. MoBULA HYPOSTOM.^ (Page 453). 



A course of evolution resembling the actual course traversed by Mobula from an ancestral form like 

 the Dasj^bati may be traced through iVIyliobatis, Aetomylaeus, and Rhinoptcra by means of the pectoral 

 and the cephalic fins — connected and meeting in front of the head and lacking the modified radials 

 opposite the gills in Myliobatis, disconnected at tlie sides but still meeting in front and possessed of the 

 modified radials in Aetomylaeus and Rhinoptera — or by means of the propterygial bases of the pectorals, 

 or even by means of the outer branchial rays their attachments and their transformations quite as readily 

 as by means of the dentition, the narial cartilages, or the skull itself. The cranium of Mobula is broader 

 and more indented in the forehead than that of Rhinoptera. The cephalic fins are distant from one 

 another in front of the head ; they are radically separated from the pectorals, the anterior rays of which 

 latter have undergone considerable changes of form. The mouth is widened; the jaws are elongate. 

 The outer branchial rays are transformed into braces or stays. Is, bra, bre, in a more firm attachment of 

 the gill arches. The extrabranchials, sbr, are highly developed; originally they were branchial rays, 

 and they do not closely correspond with the extrabranchials seen, on Plate 51, fig. 2, or on Plate 62, 

 exbr and sbr, in the shark. The stays Is, bra, bre originated as noted under Plate 74 for those in Rhinop- 

 tera. The second ray from the outer stay is lengthened, wider outward, and has a .slender curved 

 extremity; it also serves as a brace. Nearly all of the rays on the ceratohyals, chij, are segmented and 

 more or less changed in form. The long strips of cartilage, evs, first appearing as small lumps and later 

 fusing, upon the bends in the gill lamellae, parallel with the gill arches, are adventitious and are first 

 noted in Myliobatidae, Plate 73; they are named epitropeal cartilages, the upper supratropeal, the 

 lower sublropeal. The opercular cartilages, op 1-5 attain their maximum development in this family. 



