THE OSTEOLOGY OF AMIURTJS CATUS. 275 



parasphenoid lies below the basioccipital behind, and also passing 

 forwards, the prootic, basi sphenoid, orbitosphenoid and ectethmoid. 



9. Basisphenoid. 



Does not appear as a distinct bone in the skull of Amiurus but is 

 anchylosed with the subjacent parasphenoid, the line of demarcation 

 between the two being more or less distinct however. It is a flat- 

 tened impair bone, presenting no especial features for examination. 

 It forms the lower boundary of the foramen for trigeminus and 

 facialis behind, and partly of the foramen for opticus in front, and 

 articulates behind with the prootic ; externally with the alisphenoid ; 

 in front with the orbitosphenoid ; and below with the parasphenoid. 



10. Alisphenoid, (PL II. Fig. 2, AS.) 



A rather small bone, lying on either side between the foramina, of 

 which it forms the anterior and posterior boundaries, respectively. 

 These foramina are that for the trigeminus and facial behind, and 

 that for the opticus in front. The bone is very roughened and 

 ridged on its external face for the attachment of muscles, and above 

 this roughened portion is a hollow in which lies the anterior portion 

 of the hyomandibular. The inner surface is smooth. From the pos- 

 terior edge a spicule of bone passes backwards in those individuals, 

 in which the foramen for passage of the 5th and 7th nerves is divided 

 completely, which spicule unites with a similar one from the prootic. 

 Immediately in front of posterior edge is a small foramen for exit of 

 the ciliary trunk of the 5th nerve. On the inner side, immediately 

 above the inferior process, which articulates with the basisphenoid, 

 are two foramina, one above the other. The inferior of these is the 

 larger, and opens into a canal, pursuing a course more or less oblique 

 in different individuals to the exterior. It gives passage to the deep 

 branch of R. ophthalmicus trigemini. The smaller one lies at the 

 extremity of a longitudinal groove, and opens into the interior of the 

 bone like other similar foramina which, perhaps, have a nutritive 

 function. Each alisphenoid articulates above with the sphenotic 

 and frontal ; behind with the prootic ; below with the basisphenoid.; 

 And in front with the orbitosphenoid. 



11. Frontals, (PI. II. Figs 1 & 2, Fr.) 



Axe flat plate-like bones, with a small ridge projecting downwards 

 from the middle of the under surface. They are separated from each 



