124 ILLINOIS STATE ACADEMY OF SCIENCE 



by a narrow internasal space, extend upward a short dis- 

 tance and curve laterally and ventrally to cover the an- 

 terior portion of the nasal organ (Figs 1,2). This struct- 

 ure is a complete cupola and forms the anterior wall of 

 the chondrocranium. It is connected ventrally in front 

 with the dorsal margin of the cornu trabeculae, above 

 described, its upper margin extending laterally a con- 

 siderable distance to form the anterior wall of the naris 

 (Fig. 1). Lateral to the narial opening, the margin of 

 the cupola extends ventrally and terminates in a promi- 

 nent process, so that a broad deep notch is included be- 

 tween it and the lateral margin of the cornu trabeculae 

 (Fig. 1). 



The roof of the capsule, formed, as above indicated, 

 from the septum nasi and continued anteriorly with the 

 cupola, is known as the tectum nasale (Fig. 2). It con- 

 tinues laterally, and inclining somewhat ventra-poster- 

 iorly, is reduced to a narrow plate of cartilage which 

 forms the lateral wall of the capsule in this region. 

 The remaining portion of the lateral wall is continuous 

 with that portion just described, and consists of a broadly 

 curving plate of cartilage, the planum lateralis, which 

 entirely covers the lateral surface of the posterior part 

 of the nasal organ (Figs. 2, 3). Its anterior ventral mar- 

 gin is curved conspicuously and forms a prominent pro- 

 cess which lies slightly lateral and considerably below 

 the ventral margin of septum nasi. This process has an 

 intimate relation to the vomero-nasal organ, as shown 

 in Figure 5, and is known by Gaupp as the Capsule of 

 Jacobsen's Organ. 



Two large fenestrae occur within the walls of the cap- 

 sule. Of these one is dorsal, lying back of the posterior 

 margin of tectum nasale between palnum lateralis 

 and septum nasi; while the other is of irregular shape 

 and lies on the floor of the capsule, continuous with the 

 lateral narial aperture (Fig. 3). Thus the nasal organ, 

 of this stage, is exposed upon its entire ventral and pos- 

 terior dorsal surface. The anterior part of planum lat- 

 eralis is pierced by a single foramen, through which the 

 profundus branch of the trigeminal nerve passes to the 

 anterior region of the snout. 



