In the rays the pre-spiracular cartilage is of large size, and is 

 attached by short, slender ligaments to the auditory cartilage and to 

 the anterior surface of the distal end of the hyomandibular. That 

 is to say, if we regard the pre-spiracular cartilage as a chondrification 

 in the middle of a ligament extending from the auditory capsule to 

 the lower end of the hyomandibular, this ligament has exactly the 

 relations claimed for the so-called pre-spiracular ligament of the dog- 

 fish ; and Parker's error may find its explanation in this fact, seeing 

 that his observations on the skulls of the skate and dogfish were 

 published together. 



The spiracle of Elasmobranchs is not a simple tube passing from 

 the pharynx to the surface of the body, but it is complicated by 

 various outgrowths. In many genera of Selachians there is a ca3cal 

 diverticulum (figs. 1 and 2 dc) on the dorso-internal wall of the 

 spiracle, which passes inwards, and, broadening in an antero-posterior 

 direction, becomes firmly attached to the auditory capsule, below the 

 projecting ridge that marks the position of the horizontal semicircular 

 canal, and immediately above the post-orbital groove. It is, in fact, 

 only the lower part of this groove that is occupied by the post-orbital 

 blood-sinus, the spiracular caecum filling the upper part (fig. 2 pos 

 and dc). The caecum and blood-sinus are separated by a very thin 

 membrane. The opening of the caecum into the spiracle is never 

 closed, although in some genera, e. g. Galeus, it may be very 

 small. 



In the common dogfish (Scyllium) the caecum runs over the 

 outer face of the superior post-spiracular ligament, and its inner 

 caecal extremity is bounded in front by the upper insertion of this 

 ligament. The proximal portion of the caecum is bounded above by 

 the levator maxillae su peri oris muscle, and below by the 

 proximal head of the hyomandibular. The hyoidean branch of the 

 seventh nerve passes below or internal to the ca3cum. 



This caecum was first described by Müller ^), who casually ob- 

 serves that it is probably an accessory of the auditory organ. Gegen- 

 BAUR^^), and Van Bemmelen ^^) who paid considerable attention to 

 this and other spiracular structures, both accept Müller's deter- 

 mination of its function. Müller described the caecum as present in 



9) Abh. Aka(i Wis?. Birlin 1839 (publ. 1841). p. 253. 



10) 1. c. 



11) Mitt. Zool. Stat. Neapel, Bd. VI. 



