1916] Danid : Tlie Anatomii of Hrpta)ichus ))iaciilafus 357 



median two of them look very much like the rays above described 

 and they are much like them also in that they are of clear hyalin 

 cartilage. Further toward the median line on the middle piece there 

 is clear evidence of another group similar to these, excepting that 

 it is not separated into rays or lamellae. This group is also of clear 

 hyalin cartilage differing distinctly from the median piece, which is 

 a dark color in the specimen. I have interpreted this as a remnant 

 probably having to do with a ninth arch (ar."), although I am not 

 certain what part it represents. 



The condition found in this specimen is suggestive as to the method 

 of formation of the enlarged median piece so characteristic of the elas- 

 mobranchs. It would appear that in this regiim the rudimentary arches 

 are forced more and more to take a longitudinal direction nearer the 

 middle line, and that the median piece represents in its most posterior 

 part the fusion of these arches from side to side. 



Well-developed cartilaginous branchial rays are present on the 

 epibranchial and ceratobranchial segments of all of the l)ranclual 

 arches (see b.r., text-iig. A) excepting the last. These rays, although 

 much simpler than are those of the hyoid arch (cf. pi. 28. fig. 3), have 

 the same function to support the gill-septa. 



The extraviseeral cartilages in Heptanehus maculatus, although 

 well developed, are not so pronounced as those of Heterodontus 

 (Daniel, loc. cit.). The labial cartilages in the notidanids are of great 

 interest since here they are found in a comparatively simple condition 

 and may be traced in series. In Heptanehus einereus they were 

 thought to be absent until Fiirbringer (1903) discovered a single small 

 cartilage on each side lying against the upper jaw. In Heptanehus 

 maeulatus each labial (/., fig. 3) appears in a .slightly higher stage of 

 development. It is an irregular cartilage of considerable size, shaped 

 in general like a tuning-fork, the two prongs of which lie against the 

 upper jaw. and the single stem extends backward and downward 

 again.st the lower jaw. In this, so far as I can find, there is no 

 indication of a separation into parts. Only a little more complex 

 than this is that figured for Hexanchus by Gegeubaur (1872. pi. 10, 

 fig. 1), in which the labial is perforated by a foramen and separated 

 longitudinally into two parts. From some such separation the two 

 dorsal and single ventral labials characteristic of pentanehid .sharks 

 evidently have arisen. 



An extraviseeral cartilage is present dorsally over the hyoid arch 

 (ex.h., pi. 28, fig. 3), but ventrally such a .segment appears to be ab-sent. 



