NEKVES OF FISHES. 297 



even accompanies the cerebral nerves in their exit from the skull 

 in some fishes with large nerve-foramina. There is much cellular 

 arachnoid above the cerebral lobes in the Lepidosiren. A large 

 arachnoid is abundantly interposed between the dura mater and 

 pia mater in the Turtle ( Chelone), where two ligaments converge 

 Irom the arachnoid at the sides of the epencephalon to be attached 

 to a cartilaginous tubercle on the basioccipital. A number of 

 filamentary processes pass from the space between the cerebral 

 and optic lobes to the arachnoid above, like a rudimentary 

 ' falx.' 



The primitive fibrous capsule of the neural axis, the unossified 

 or unchrondrified remains of which, or of its inner layer, form 

 the so-called ' dura mater,' is most distinct in the low-organised 

 Dermopteri ; in the Plagiostomi it is reduced to a few thin shining 

 aponeurotic bands closely adherent to the inner surface of the 

 cartilaginous walls of the cranium and spinal canal ; svich traces 

 of dura mater are more feeble and indistinct in Osseous Fishes, 

 in which no proper continuous fibrous membrane can be dis- 

 tinguished from the inner periosteum of the walls of the cerebro- 

 spinal cavity : no curtains of dru-a mater divide the cerebral from 

 the acoustic compartments of the cranium in the Osseous Fishes. 

 The dura mater, as a distinct fibrous membrane, lines the cavity 

 of the skull and spinal column in Reptiles. 



§ 55. Nerves of Fishes. — First pair or Olfactory nerves. — The 

 head is short and obtuse in the embryo fish ; the ganglionic 

 centres of the olfactory nerves are always originally developed 

 in close contiguity with the prosencephalon ; they are protected, 

 pi-imarily, by the rhinencephalic arch ; and, as this advances 

 in the elongation of the skull, and recedes from the prosence- 

 phalic arch, two modes of growth take place in the contained 

 nervous axis : either the brain is co-elongated, the rhinence- 

 phalon retaining its primitive relation with its vertebra, and the 

 prolonged crura occupying the narrow interorbital tract of the 

 cranial cavity, or the rhinencephalon retains its prunitive juxta- 

 position with the prosencephalon, and the olfactory nerves, figs. 

 180 — 182, 0, 203, 0, are prolonged through the interorbital space, 

 perforate or traverse a notch in the prefrontals, and expand, as a 

 resolved plexus, upon the pituitary plicated sac. 



The rhinencephalon accompanies its vertebra and recedes from 

 the rest of the brain in Salmo, Cyprinus proper, Brama, Tinea, 

 Gadus, Lota, Hippoglossus, Clupea, Belone, Lucioperca, Cobitis, 

 Plectognathi, and Plagiostomi; it retains its primitive contiguity 

 with the prosencephalon in Perca, Scomber, Esox, Pleuronectes, 



