200 AYERS AND JACKSON. [Vol. XVII. 



number of arches seems, in general, to vary according to the 

 size of the specimen, but this is by no means always the case. 

 In one instance a very small specimen (about fifteen inches 

 long) possessed ten nasal arches. In an 8-arched form the 

 fifth arch was unusually wide, and was divided in the dorsal 

 portion by a small slit {Fig. a), indicating the fusion of two 

 arches. In a lo-arched form a rudimentary eleventh arch was 

 represented by a lateral piece between the sixth and seventh 

 arches on the right side, which extended dorsally nearly to the 

 median line. Below, it fused with the seventh arch near the 

 end. On the left side in this same (seventh) arch the lower 

 end was bifurcated. In the same specimen there was a short 

 process projecting backward from the sixth arch, a little to 

 the right of the median line. In a 9-arched form the fifth 

 and sixth arches were connected by a median longitudinal bar. 

 In another 9-arched form the fifth and sixth arches were 

 partly fused together dorsally, a small slit separating them in 

 the median line, as shown in Fig. b. In a lo-arched form the 

 sixth and seventh arches were somewhat similarly joined on 

 each side of the median dorsal line, as in Fig. c. The last 

 two arches were always found joined in the median dorsal line. 

 Usually this was accomplished by a short longitudinal bar, as 

 in Fig. d\ but in several cases the two arches were partly 

 fused, as in Fig. e, and once as in Fig./. The anterior arch 

 is wide and thin, usually perforated by from two to four circu- 

 lar foramina on the dorsal side. In one case the arch was 

 found unperforated. It was usually smooth on the anterior 

 margin, but in one case it was found marked by several irregu- 

 lar notches. The lateral plates of the olfactory capsules were 

 found in a few instances perforated by two foramina, but 

 usually unperforated. They vary considerably in shape and 

 extent, as previously stated. Having completed the descrip- 

 tion of that part of the skull which constitutes the smaller 

 dorsal tube, we shall now consider those structures which 

 surround and support the larger ventral tube. In the antero- 

 lateral region, just below the level of the floor of the nasal tube, 

 are two bars, the cornual cartilages (PL XXII, Figs. 5, 6; 

 PI. XXIII, Fig. 7, cc). They are a pair of slender cartilaginous 



