62 Journal of Comparative Neurology. 



Acipenser and Mustelus. The Purkinje cells, according to 

 Houser's statement, have the dendrites directed toward the ex- 

 ternal surface ; if so. those lying on either side of the deep 

 groove must be directed toward one another as in Acipenser 

 (cf. Houser's Fig. 2 with my figures in '98 b). This explana- 

 tion of the position of the acusticum, lobus lineae lateralis, and 

 cerebellar crest in selachians and ganoids is rendered entirely 

 clear when the brain of Petromyzon is taken into account, 

 where the lobus is seen to be only a special collection of cells 

 in the dorsal part of the acusticum and the cerebellar crest 

 covers the outer surface of the acusticum as the molecular layer 

 of the cerebellum covers the granular layer. 



The direct continuity of the ^. c. n. with the dorsal 

 horn, the ending of general cutaneous fibers in it, and the struc- 

 ture of the nucleus itself all agree with the tuberculum acusti- 

 cum in Acipenser and Petromyzon. Houser, indeed, says that 

 "Johnston ('98 b) appears to include a part of this region 

 under his tuberculum acusikum " (p. 95). In describing the 

 tuberculum acusticum he says: "The term trigeminal lobe has 

 been so variously used that it should be dropped from our 

 nomenclature. The earlier writers on the brain of the selachian 

 so designated the tuberculum acusticum " etc. (p. lOO). From 

 this it appears that Houser has consciously adopted new names 

 for both these structures. The reason for this is not given, but 

 it must lie in his conclusions regarding the central endings of 

 the general and special cutaneous nerves. He represents the 

 trigeminus alone as ending in the so-called general cutaneous 

 nucleus, while the acustico-lateral fibers all enter his tuberculum 

 acusticum through the narrow neck between the cerebellar crest 

 and the ventricle (see his Fig. 2). In Acipenser and Petromy- 

 zon the large special cutaneous nerves plunge directly into the 

 body of what Houser calls the general cutaneous nucleus and 

 the bundles of fibers running forward and backward in that 

 center and the endings of fibers are readily made out. Houser 

 does not describe the point of entrance of any of these nerves, 

 but says that they reach the acusticum as arcuate fibers from 

 the opposite side. From this it seems probable that he has not 



