OSTEOLOGY ETC., OF SYNGNATHUS PECKIANUS. 625 



while above these is a cartilaginous plate (Tg. Cr.) extending 

 upwards to between the eyes, and produced to a point between 

 the nares, forming the tegmen cranii. This and the two 

 nasal rods have apparently no connection at present with the 

 coalesced trabeculae, but are apparently independent cartilages: 

 the tissue lying between the nares and the extremity of the 

 trabeculse is somewhat fibrous. The auditory region is now 

 more fully developed, and the semicircular canals are repre- 

 sented by fibrous bands. The parachordals posteriorly have 

 extended upwards and around the nervous cord to form the 

 occipital region of the skull. 



Stages c and d are very slightly different. They present an 

 increase in size from Stage b, and the fibrous semicircular 

 canals have become cartilaginous. The snout has increased 

 somewhat in length in c, and still more in Stage d, the growth 

 taking place at first by an increase in length of the horizontal 

 portion of the cartilaginous "rostral plate," as the elongated coal- 

 esced trabeculae may be called, and, latterly, by a straightening 

 out of its bent up end, which even in Stage c (fig. 5) forms a 

 right angle with the horizontal portion. The posterior region 

 of the skull is almost completely encased in cartilage, its very 

 summit only being of dense membrane ; more anteriorly the 

 membrane becomes thinner, and extends further down the 

 sides of the skull. 



At the base of the skull, and extending from its posterior 

 portion forward for a considerable distance in the median line, 

 is a dense membrane (fig. 15, Pa. S.), identical in appearance 

 with that roofing in the cranial cavity, and also with the mem- 

 brane surrounding the notochord at this stage : this is the 

 commencement of the formation of a bone, which from its 

 position must be the parasphenoid. At the sides of the 

 vertex of the anterior portion of the cranium proper, and ex- 

 tending back nearly to its posterior portion, are two similar 

 condensations of tissue (Fr. S.). They consist, as is seen on 

 section, of plates lying in the connective tissue enclosing the 

 cranial cavity, one on either side, and at the centre of each 

 plate and perpendicular to it a ridge passes along its entire 



