138 PROF. HUXLEY ON THE OVIDUCTS OF OSMERUS. [Mar. 20, 



racters of the female reproductive organs can lend no support to any 

 attempt to draw a sharp line of demarcation between the Ganoids 

 and the Teleosteans. 



Boas' has recently conclusively shown that the same is true of the 

 supposed distinctive character afforded by the conus arteriosus ; and 

 it has long been admitted that the spiral valve which has been 

 described in the intestine of Chirocentrus^ is the homologue of that 

 ■which exists in all the Ganoids, though greatly reduced in Lepi- 

 dosteus. Indeed I am inclined to believe that the circular valve 

 which separates the colon from the rectum in the Smelt is merely a 

 last remainder of the spiral valve. Thus, among the supposed 

 absolute distinctions between the Ganoids and the, Teleostei, only 

 the peculiarities of the brain, and especially the so-called chiasma 

 of the optic nerves, remain for consideration. My lamented friend 

 Mr. Balfour, in the last of his many valuable labours, proved con- 

 clusively that the brain of Lepidosteus is, both in structure and 

 development, a Teleostean brain. But it is singular that no one, so 

 far as I know, has insisted upon the fact, not only that the Teleos- 

 tean brain is essentially similar to that of the Ganoids, but that it is 

 exactly in those respects in which the Ganoids and Teleostei agree 

 in cerebral structure that they differ most markedly from the Pla- 

 giostomi and the Chimaeroidei. 



In a communication read before this Society some years ago ^, I 

 pointed out that the parts of the brain termed cerebral hemispheres 

 in the Selachians arise in a very peculiar manner, the anterior cere- 

 bral vesicle becoming subdivided by a median anterior partition, and 

 the walls of the two ventricular cavities thus formed becoming 

 greatly thickened. The lateral walls of the undivided part of the 

 anterior vesicle also become thickened to form the optic thalami ; but 

 these give rise to no lobular outgrowths from their upper edges *. 



In the Ganoids the anterior cerebral vesicle undergoes a totally diffe- 

 rent series of modifications, inasmuch as no median septum is developed 

 and no lateral ventricles are produced. In the Sturiones the thick 

 lateral walls of the anterior cerebral vesicle give rise to no distinct 

 superior lobes. In Lepidosteus, however, as Balfour has shown, such 

 solid lobular bodies, or epithalami, are developed, and, giving rise to 

 a thickened decurved overlapping rim from their outer faces, become 

 exactly similar to the so-called " cerebral hemispheres " of the 

 Teleosteans. In all the Teleosteans, in fact, the bodies called " cere- 

 bral hemispheres " are not the exact equivalents of the structures so 

 named either in the higher Vertebrata or in the Selachians, but are 



^ " Ueber den Conns arteriosus bei Butirinus und bei anderen Knochen- 

 fiscben," Morpbol. Jabrbuch, yi. 4, 1880. 



^ Doubts have been thrown on the existence of this structure in Chirocenirus ; 

 so that the matter needs reinvestigation. [By the kindness of Dr. Day I have 

 been enabled to examine a small specimen of Chirocentrus clorab ; and I find it 

 to possess just such an intestinal valve as that figured by Valenciennes. Whether 

 it is truly "spiral" in its arrangement, or not, can only be detei-mined by the 

 examination of a larger specimen. — T. H. H., July. 1, 1883.] 



' " On Ceratodusforsteri," Proc. Zool. Soc. January 4, 1876. 



* See Balfour, ' Development of the Elasmobranchs.' 



