OEGAN OF THE SYLUEOID HYPOPHTHALMUS. Ill 



to be afterwards described. The exoccipitals, besides forming a considerable part of the 

 floor of the skull (where they are perforated by the glossopharyngei), likewise contribute 

 to the formation of the hinder wall and sides. Instead of the foramen for the vacrus beinjr 

 in the floor as in Amiurus, it is in a large fenestra on the hinder wall, which is bounded 

 medially by a slender bony style, through which the first spinal (occipital) nerve escapes, 

 and which simultaneou.sly forms the antero-lateral boundary of thesaccus paravertebralis, 

 through w^hich the malleus reaches the interior of the cranial cavity. The epiotics are 

 entirely, and the pterotics partially, excluded from appearing in the inner aspect of the 

 brain case by the exoccipitals. 



Entering into the composition of the auditory capsule are the prootic, epiotic, pterotic, 

 and sphenotic. The two latter chiefly appear on the side walls of the skull, and are 

 grooved by the fossa for the hyomandibular, while the prootic is confined to the floor, and 

 the epiotic to the outer aspect of the roof. The hyomandibular and palatine branches of 

 the seventh nerve pierce the prootic, which likewise contributes to the formation of the 

 trigeminal foramen. 



There is no opisthotic. Both pterotics and sphenotics are largely formed round the neu- 

 romastic (sensory) canals, which are continued forwards in these bones from the snpraclavi- 

 cles Although the medial borders of theprootics are in contact with each other in the floor 

 of the skull for their posterior halves (being there covered by the parasphenoid), they are 

 separated in front by a backward projection of the basisphenoid, between which and the 

 basioccipital a considerable amount of cartilage persists in the adult skull. The basi- 

 sphenoid and alisphenoids contribute with the prootics to the formation of the trigemi- 

 nal foramen, and the former, as in all the Nematognathi I have examined, is fused with 

 the parasphenoid, and comes away with the latter in the disarticulated skull. Between 

 the basi- and ali-spheuoid on each side is the small foramen for the otitic nerve. 



The alisphenoids are only visible on the floor of the skull, they form with the sphen- 

 otics the posterior boundaries of the orbits and in addition to articulating with the basi- 

 sphenoid and sphenotics, form the chief surface of union between the unpaired orbito- 

 sphenoid and the middle region of the skull. 



Covering the orbitospheuoid in the middle line is the anterior half of the para- 

 sphenoid, which lies in a groove on the under surface of the former, and is readily 

 detached from it by maceration. The parasphenoid presents similar features to the same 

 bone in Liposarcus (Hypostomidœ), and it appears to me probable that both it and the 

 vomer occur in all the members of that family, and that Goldi overlooked them in Lori- 

 caria on account of their slenderness. 



Only a very small portion of the orbitospheuoid appears on the upper surface of the 

 skull, and this part is exposed by the divergence anteriorly of the frontals, which are in 

 contact (by a deeper lamella of thin osseous substance) from the cranial fontanelle already 

 referred to, as far as another and smaller fontanelle which corresponds to the foremost part 

 of the cranial cavity, and is to be seen directly behind the orbitospheuoid in Fig. 1. 



The frontals are sculptured on the greater part of their upper surface ; they form 

 ledges overhanging the orbits, and articulate with the orbitospheuoid for only a small 

 extent, in front of which a slit (indicated in Fig. 2) separates the two bones. 



In front of the orbitospheuoid and frontal are the parethmoids, which for part of their 

 upper surface partake of the sculpturing of the frontals. They are in contact with each 



