THE ELASMOBRANCH FISHES 57 



The middle region varies greatly in dorsal view. While it is typically con- 

 stricted, secondarily this constriction may be obscured by the expansion of the 

 supraorbital crests (Hetei^odontus, fig. 61), or the constriction present in the 

 young may be entirely lost in the adult, as in Carcharodon rondeletii (T. J. 

 Parker, 1887). In the rays a snpracranial fontanelle is usually present in the 

 middorsal line. This fontanelle is small in Trygonorhina, but large in Rhino- 

 hatis {F}, fig. 62) and Baja clavata. 



The olfactory capsules {ol.c, fig. 60) which represent the uppermost seg- 

 ment of the hourglass are in most Elasmobranchs more or less well developed. 

 In some of the sharks they may be relatively of immense size, as for example 

 in Zygaena where they are drawn far out with the preorbital processes of the 

 cranium (fig. 65). But the capsules are normally smaller as in Pseudotriacis 

 (fig. 59) . In Rhinobatis (fig. 64) and many other rays they are large ^ang-like 

 expansions to which heavy antorbital pieces are attached. In the middorsal 

 line of this median segment there is present in all sharks and rays the anterior 

 fontanelle (F.), which in some is confluent with the snpracranial fontanelle 

 (Myh'ohatis) . 



The rostral pieces which may form an apex to the hourglass may be flat and 

 divergent, as in Heterodontus (fig. 61) and Crossorhinus, or blunt and con- 

 vergent as in Scyninus and Laemargus; or there may be two long dorsolateral 

 bars which unite forward with a ventral bar as in Pseudotriacis microdon (vs., 

 fig. 59) , Mustelus, Galeus, and many others. In Acanthias the ventral cartilage 

 is broad and spoon-shaped and the dorsolateral bar is rudimentary, each bar 

 being joined to the nasal capsule by a double cord of connective tissue (Wells, 

 1917). In some of the rays (Myliohatis) a rostrum is lacking, but this is the 

 exception. In the majority of this group it is a long single piece (Rhmobatis, 

 fig. 62) , and in some it is of great length, as for example in Pristis the saw ray. 



In a ventral view of the cranium (figs. 60, 68, and 64) a similar hourglass 

 shape is apparent. The auditory region at the base is a broad expansion of the 

 parachordal cartilage of the embryo to which the auditory capsules have 

 fused. In the most posterior region of the midventral line is the notch separat- 

 ing the two occipital condyles (ocd., fig. 60) ; by means of the condyles the 

 spinal column articulates with the cranium. In primitive sharks like Hep- 

 tancluis the column is more or less completely fused with the cranium so that 

 no true articulation exists. In the rays the condyles are well developed, but 

 here a secondary fusion takes place which afi^ects the occipitovertebral articu- 

 lation and the anterior part of the column (see p. 70, fig. 76b) . Posteriorly and 

 between the two condyles is the foramen magnum {f.m., fig. 61) through 

 which the spinal cord joins the brain. 



The middle segment in ventral view is characteristically different in the 

 sharks and rays. In the adult of most of the sharks right and left infraorbital 

 plates broaden out into wing-like processes which form a floor for the orbits, 

 and consequently ol)scure the constriction. In the rays the form of the hour- 

 glass is very evident, for in them, as in Heptanchus, an extension of these 

 plates is characteristically absent. 



