THE SKULL OF ACANTHIAS VULGARIS 419 



tical above and then curve downwards and medially to form the 

 floor of the trough. Behind, the fontanelle is bounded by the 

 thin cartilaginous roof of the brain case. The anterior margin 

 of this roof has a deep median notch, and separated from this, 

 on either side, by a slender triangular process, is a lesser notch, 

 the lateral boundary of which passes into the lateral wall of the 

 rostrum. The trough of the rostrum is filled with a loose gelati- 

 nous tissue. 



On either side and extending outward and posteriorly from 

 the rostral carina is a small cartilage process, the lateral rostral 

 bar (r.b.) the 'anhang' of Gegenbaur. Farther back this carti- 

 lage passes into a dense gelatinous connective tissue with rather 

 definite boundaries, appearing as dorsal and ventral rami (dr. vr.) 

 continuous with the anterior cartilaginous rostral bar, and ar- 

 ticulating with the dorsal and ventral walls of the nasal capsule. 

 The extent of cartilage was determined by a study of sections, 

 both longitudinal and transverse, through these bars which were 

 previously stained with borax carmine. 



Apparently these rostral bars have not been noticed before 

 in Acanthias. Evidently they are the same as the single bar 

 labelled V by Gegenbaur in his figures of Galeus, Mustelus, 

 Prionodon, Scyllium and Pristiurus on his plate VIII, and it 

 would seem that they are represented as rudiments in his fig- 

 ures of Centrophorus and Zygaena, there lettered 'r.' If this 

 homology be true it follows that what is here called the rostrum, 

 is Gegenbaur's "Medialer Schenkel des Rostrum," (R), in his 

 figures. In Zygaena the case appears somewhat different, 

 where, owing to the great lateral development of the sides of 

 the skull, carrying the eyes out to the end of the 'hammer/ the 

 nasal capsules are far removed from the median line. In this 

 form the rostrum proper is composed of three bars, these form- 

 ing the rudimentary sides and floor of the trough. Beside this 

 there is the lateral ' anhang' (r') of Gegenbaur, which would thus 

 correspond to the rostral bar of Acanthias. 



The postero-lateral part of the cranium is formed by the otic 

 capsules, a postorbital process extending laterally from the an- 

 terolateral angle of each. The foramen magnum (f.m.) for the 



