152 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



median roof of the mesencephalon. This relation is especially clear in 

 such forms as Amia and Ameiurus. The sinus covered by this plexus is 

 much wider in P. planeri than in P. marinus, so that P. marinus repre- 

 sents a condition in this respect intermediate between that of P. planeri 

 and Amia. 



There is no structure in the brain of Petromyzon homologous to the 

 torus longitudinalis of tlie brain of teleosts. Ahlborn has represented, 

 in the brain of Petromyzon planeri (Figs. 23, 24), a median down- 

 folding of the mesal edge of each half of the tectum opticum, forming a 

 pair of longitudinal ridges projecting downward iiito the posterior part 

 of the mesocoele. As seen in Ahlborn's figures, the resemblance of 

 these ridges to the torus longitudinalis in teleosts is striking. They 

 cannot, however, in any way represent the torus longitudinalis of higher 

 forms, for these ridges are found only posterior to the plexus chorioideus 

 II. In P. marinus they are but little developed. They more nearly 

 correspond in position with the corpora bigeraina posterior. The tori 

 semicirculares, projecting into the mesocoele from the lateral walls of the 

 optic lobes, are well developed, but are not so prominent in P. marinus 

 as Ahlborn has figured them (Figs. 24, 25) for P. planeri. 



The posterior commissure crosses through the roof of the mesenceph- 

 alon in a caudo-ventral downfolding of the mesencephalic roof (Fig. A). 

 The mesocoele is prolonged dorsad and anteriad of the posterior com- 

 missure into a blind pocket, or recessus, which is continued cephalad 

 in two lateral horns (Plate 1, Figs. 6, 7). The ependymal prolifera- 

 tion ventral to the commissure, which in gnathostomes is median, in P. 

 marinus, following the bilateral division of the roof of the mesocoele, is 

 in two lateral divisions. This thickened ependyma forms two longi- 

 tudinal grooves (^niJ. L een<l., Figs. 6, 7), one on either side of the 

 median plane, extending along the whole length of the roof of the dia- 

 coele ; that of the right side is somewhat the larger, and is better 

 developed, especially at its anterior end, corresponding in this with the 

 greater size of the habenula of that side. Anteriorly, the ependymal 

 grooves reach to the base of the ganglia habenulae. Posteriorly, both 

 grooves extend along the roof of the diacoele and downward under the 

 posterior commissure, at the same time coming nearer together. Here 

 they curve around the commissure, which crosses the brain in a down- 

 folding of the roof, and continue cephalad into the recessus of the 

 mesocoele above the commissure, and thence into its anterior horns. 

 The horns of the recessus are completely lined by this characteristic 

 ej^endyma. A transverse section through the anterior part of the pos- 



