1 94 AYERS AND JACKSON. [Vol. XVII. 



of cartilages known as the "pharyngeal basket"; while in 

 the gill region the series of small gill bars represents the 

 remarkable " branchial basket " of Petromyzon. 



The membranous cranium (PI. XXII, Figs. 4-6, cr) is a 

 direct continuation of the skeletogenous layer surrounding the 

 spinal cord {cf. Figs, i and 4). The "fatty tissue" is also 

 continued into the cranial cavity as a thick layer of loosely 

 interwoven elastic fibers (P). The cranium is somewhat flat- 

 tened dorso-ventrally. 



The anterior wall has a median dorso-ventral ridge which 

 projects backward between the olfactory lobes of the brain. 

 The floor is concave within, both laterally and longitudinally. 

 The roof projects upward in a median longitudinal septum 

 (Fig. 4, mds) between the myotomes of the right and left 

 halves of the body. The cranium is perforated laterally for 

 the exit of all the cranial nerves, except the olfactory, which 

 perforate the anterior wall. The cranial walls have no carti- 

 laginous support, excepting the floor, which is supported by 

 the parachordal cartilages and the trabeculae. 



The parachordal cartilages (PI. XXII, Fig. 4; PI. XXIII, 

 Fig. 7, pc) are a pair of cartilages which support the posterior 

 portion of the cranial floor {os basilare of J. Miiller). The para- 

 chordals are composed chiefly of a pair of roughly prismatic 

 cartilaginous bars lying in the skeletogenous layer of the mem- 

 branous cranium. The posterior halves of the parachordals are 

 situated one on each side of the notochord. They are nearly 

 parallel, converging slightly toward the median line, from behind 

 forward. The anterior halves of the parachordals diverge, and 

 extend antero-laterad to fuse with the posterior ends of the 

 trabeculae. The diverging anterior halves of the parachordal 

 bars form the posterior boundary of the large hypophysial fon- 

 tanelle (see Fig. 7). In the angle formed by these diverging bars 

 the cartilaginous anterior tip of the notochord projects in the 

 median line, as previously noted. In the posterior region the 

 parachordal cartilages fuse together in the median line under 

 the notochord, so that the latter appears to lie in an open 

 groove, as shown in cross-section in Fig. 4. In some places, 

 however, the parachordal cartilages fuse together both above 



