429 



Gegenbaur in 1872 ^) correctly figured the pre-spiracular cartilage 

 of Mustelus and Galeus'^) as distinct from, and in front of, the 

 ligament, but his description does not state whether the ligament is 

 in front of or behind the spiracle. 



The pre-spiracular cartilage is described by Gegenbaur as 

 occurring in two pieces in Scymnus and Acanthias, in three in 

 Centrophorus, and in four pieces in Torpedo. As far as I 

 have been able to make out, the pre-spiracular cartilage is absent 

 in those few Selachoids (the Laranidae and four genera of the Car- 

 chariidse) in which the spiracle is closed. Its absence however in 

 these is rather due to a process of reduction and ultimate disappear- 

 ance of the separated cartilage, than to its failure to separate from 

 the palatoquadrate as in Notidanus. In Care h arias ^) there is a 

 bar of cartilage arising from the auditory capsule, above the level 

 of the horizontal semicircular canal and behind the lower extremity 

 of the ridge that marks the position of the anterior vertical semicircular 

 canal. It passes outwards, downwards and forwards, and terminates 

 immediately behind the eyeball. It is quite free from the palato- 

 quadrate and runs over its upper edge. It passes over, i. e. external 

 to the levator maxilla3 superioris muscle, and cannot be re- 

 garded as the homologue of the pre-spiracular cartilage. In fact it 

 does not belong to the visceral skeleton at all, but is simply a short 

 representative of that long cartilaginous bar which arises laterally 

 from the auditory capsule inZyga3na. InGaleus, another member 

 of the Carchariidse, this cartilaginous projection of the auditory car- 

 tilage is also present. It arises nearer the roof of the skull than in 

 Carcharias and Zygsena, but, as in these genera, it accompanies 

 the mucous tube connecting the tegumentary canal systems of the 

 dorsal and lateral surfaces of the head. In Gal ejus the true pre- 

 spiracular cartilage is present in addition. A representative of this 

 process can be seen in the common dogfish, Scyllium canicula, 

 in the blunt projection, perforated in this genus by the mucous canal, 

 at the antero-external border of the auditory capsule (see fig. 1 c). 



6) 1. c. 



7) Fig. 2, Taf. XII, is described in the "Erklärung der Abbildungen" 

 as the skull of Gale us, and Fig. 3, Taf. XI, as that of Mustelus. 

 The characters of the teeth, however, lead one to suspect that the names 

 have been interchanged. The figures, moreover, are referred to in the 

 reverse order in the text (p. 199). 



8) The specimen examined was a young one: the relations may be 

 slightly different in the adult. 



