102 



STUDIES ON THE ELASMOBRANCH SKELETON, 



any common groove or depression. The articular surface for the 

 hyoid is an antero-posteriorly elongated surface placed close to the 

 base of the skull. In front of it is a wide bridge of cartilage for 

 the protection of the facial. Behind it are the two small 

 articular facets with which the hyoid and first branchial arches 

 articulate. These are borne on a rather conspicuous ridge, which 

 in front forms the posterior and upper boundary of the articular 

 surface for the hyo-mandibular, and behind is continuous with the 

 processes bearing the occipital condyles. The post-orbital process 

 takes the form of a long and broad, thin lamina of cartilage 

 stretching outwards and forwards from the post-orbital region of 

 the roof of the skull. The cavity of the cranium dilates con- 

 siderably in front towards the region of the olfactory capsules, 

 which are greatly elongated transversely and connected together 

 in the middle line. The prse-frontal foramen is very wide, and 

 is continuous behind with the supra-cranial fontanelle. The 

 praeorbital process has the form of a short pointed projection- 

 The ant-orbital cartilages are vertically compressed, blade-like 

 laminae articulating with the lateral angles of the olfactory capsules 

 and directed backwards and outwards. 



The hyo-mandibular is made up, as in most Rays, of two distinct 

 segments, of which the distal is much the smaller. 



In Trygon, as in many of the Batoidei, the ventral elements of 

 the hyoidean arch are not connected with the hyomandibular. 

 In Trygon pastinaca they articulate directly with the periotic 

 region of the skull just behind the surface of articulation for 

 the hyo-mandibular. The hyoid is intimately connected with 

 the first branchial arch, and the latter in turn also articulates 

 with the auditory region of the skull behind the articulation for 

 the former. 



The hyoid arch, as in Rays generally, has but a slender copula, 

 and articulating with the ventral extremity of each ventral 

 mesial is a flattened bar of cartilage, which passes almost straight 

 forwards to end in a free extremity ; this seems to represent the 

 copulare. 



