268 



IOWA GEOLOGICAL SURVEY 



here viewed from the dorsal aspect (Fig. 40 B 1 ). The brain, 

 to be seen in its natural position within the interior of the head, 

 Fig. 40 A, is oriented in a corresponding fashion, and the nodule 

 containing it is fractured in such manner as to cleave off the 

 cranial roof, which latter is seen from the cerebral aspect in A 1 . 

 Both specimens, A and B, show a relatively large-sized fish 

 brain, as completely formed as in any modern teleost, and seg- 

 mented in the usual manner. 



In the median line posteriorly is to be observed the funnel- 

 shaped expansion of the spinal cord known as the medulla, 

 which merges forwardly into a quadripartite organ well repre- 

 sented at the base of Fig. 40 B 1 , which can scarcely be inter- 

 preted other than the cerebellum or hindbrain. This organ is 

 moderately large, probably correlating with the activity of the 

 animal, just as the large size of the optic lobes is correlated with 

 a keen visual sense, and the auditory ampullae with a well de- 

 veloped faculty of equilibration. The two anteriorly situated 

 lobes of the cerebellum are larger than the posterior pair, and 

 diverge laterally so as to embrace the bilobed midbrain or mesen- 

 cephalon, whose dorsal wall is constricted into two lateral por- 

 tions known as the optic lobes. That the eyes were of impor- 

 tance for this creature is betokened by the large size of the 

 optic lobes, and also by the relatively wide orbital openings. 

 The section of the brain lying immediately in advance of the 

 optic lobes is that formed by the cerebral hemispheres, still cov- 

 ered in one of the specimens (Fig. 40 B 1 ) by a choroid plexus. 



a 



Fig. 41. 



Fig. 41. Rhadinichthys deani, sp. nov. Base of the Waverly; near Junction City, Boyle 

 county, Kentucky, a, 6 Cranial shields with partly denuded surface ornamentation, show- 

 ing course of sensory canals and parieto-frontal sutures, c Impression of internal surface 

 of cranial roof showing detail of superficial ornament. Photographic reproductions of the 

 same specimens are given in Plate XIII, figs. 5, 7, and 11. Slightly enlarged. 



