92 Journal of Comparative Neurology 



not been able to find a description of these structures by any 

 other author. Even Herrick in his late papers on the reptilian 

 brain seems entirely to overlook them. This is the more sur- 

 prising as they are in Chelydra enormously large, being much 

 larger than the nidi of the postgemina figured in all his works 

 under the name of corpora posteriora. In outline this nidus is 

 conical in form with the base caudad. This base projects 

 caudad beyond the pregemina and forms externally the repre- 

 sentatives of the " interoptic lobes " of Spitzka. These cones 

 extend cephalad about two-thirds of the distance to the cephalic 

 limit of the mesencephal. The structure of these nidi is simi- 

 lar to that of the nidi of the postgemina. Seen in transection 

 (Fig. 22) they show as a compact oval mass sharply differen- 

 tiated from the surrounding tissues by an almost open ring 

 about them (Fig. 24). The ental surface especially is formed 

 by a closely packed layer of small pyramidal cells with apexes 

 toward the central part of the nidus, while the remaining sur- 

 face of each nidus is composed of the same materials as the 

 body. This body of the nidus consists of a dense stroma, prob- 

 ably glial in character, in which are imbedded scattered pyra- 

 midal cells. What the function of this large nidus may be I 

 have not been able to ascertain. Lying laterad and slightly 

 ventrad from the caudal portion of this nidus is another of very 

 similar appearance and structure (Figs. 22-23). I ts ce ^ s how- 

 ever are not distributed in rows along the surface of the nidus 

 but are arranged in groups through the dense stroma and are 

 more sharply pyramidal in outline. The general form of this 

 nidus is irregularly conical, with the base caudad and the height 

 of the cone about equal to the diameter of the base. 



This nidus is the homolog of the postgeminum. 



TJie Tectum Opticum. The structure of the roof of the 

 optic lobes has been a very attractive subject for neurologists. 

 Its structure of concentric layers and apparent simplicity com 

 bined with its function in the sense of vision, has brought many 

 students to its study. Among those who have contributed to 

 our knowledge of this interesting region are Osborn, Herrick, 

 Nakagawa and Turner, of our own country, and especially Ca- 



