212 ME. W. K. PAEKER ON THE STETJCTTTEE AND DEVELOPMENT 



is not supplemented with an " extravisceral " cartilage. There are three pairs of labials 

 in the trabeculo-nasal region — one pair on the pterj'go-quadrate arcade, one pair in the 

 mandible itself, and a pair of similar cartilages to each branchial arch save the last. 



The first of these external cartilages, which was a small lobe in front of the nasal sac 

 in my third stage (PI. XXXVI. figs. 3, 4, 5, 1. 1), and had only one or two slime-glands 

 in it, has now been carried over the great nasal dome behind, and over the basitrabe- 

 cular in front ; it contains a row of about ten slime-glands, and forms the side of the 

 triangular " cutwater." 



The second labial [I. 2) is a thin lunate shell of cartilage, having an irregularly 

 crenate free margin ; it has partly coalesced with the trabecular comu and anterior 

 rim of the inverted nasal cup ; it lessens the size of the nasal opening. 



The third labial {I. 3) is thick, phalangiform, and twisted ; it is applied as a valve 

 to the outer rim of the nasal cup, and exactly corresponds with the " appendix alae 

 nasi " of the mammal. 



The fourth labial {I. 4) is a small pointed spatula of cartilage, applied obliquely to 

 the middle part of the outer face of the pterygo-quadrate ; its broad end is in front.^ 



The fifth labial {I. 5) is very similar, but is smaller, and is applied to the mandible 

 in such a manner as to meet, by its pointed posterior end, the fourth labial ; the angle 

 at which they meet is very acute in the closed mouth. 



The remainder of the " extravisceral " cartilages may be called " extrabranchiales ;" 

 they are four in number, and lie inside the skin of the branchial pouches at the ex- 

 tremity of the branchial rays ; thus, in the figures (PI. XXXVIII. figs. 1 & 2, e.vs 

 they are shown opposite the arch next behind. Each bar is sigmoid, pointed above 

 and pedate below. The second has the largest " foot ; " the last is rudimentary, and is 

 attached to the fifth branchial, although it belongs to the fourth. 



The reason for the absence of an extrabranchial cartilage from the gill-bearinff 



O Do 



hyoid is not evident ; they appear to belong to the same category as the labial carti- 

 lages ; hence I propose to call them all " extraviscerals." 



These extrabranchials are a source of strength to the gill-pouches ; they lie in their 

 lower part principally, their pointed end being attached to the free extremities of the 

 branchial rays, between the slits {clefts), and close in front of the slit which intervenes 

 between the arch to which that particular extrabranchial belongs and the one next 

 following. 



The extrabranchials cease as such in front of the last branchial arch; but they 

 are followed by a pair of precisely similar cartilages, which early coalesce together 

 below by their broad pedate end. These are " extracostal " cartilages ; and from them 

 proceed the rays that form the skeleton of the pectoral fins. This pair of coalesced 

 cartilages form the " shoulder-girdle," or scapulo-coracoid belt. 



' Id Scyinnus, Squatina, CentropJiorvs, and others, there is another lahial in front of this, which corresponds 

 to the fourth of the Skate. — See Gegenbaur, pis. 11 & 12, and Tl. XLII. fig. 4, I. 4, of this paper. 



