198 PEOr. W. K. PAETCER ON THE STRFCTTJEE AISTD 



B. The Endocranium of the same Larva of Triton cristatus. 

 This structure is very instructive, and is already very complete both in its general 

 finish and in the development of the osseous centres. 



The occipital condyles (oc. c) are large, oval, pedunculate, posterior, and somewhat 

 inturned. The foramen magnum is directly posterior, and both above and below the 

 skull has a large round notch between the projecting condyles ; this is filled below by 

 the large oblong " odontoid rudiment " (od. v). 



That process is seen to be due to the development of a small intercalary vertebra 

 (figs. 3, 4, od. v), the symmetrical parts of which are two small nodules of cartilage close 

 to the edge of the basioccipital investing mass (it'). 



The large space which the notochord runs through between the basis cranii and the 

 first perfect vertebra (yr') is seen to be thus utilized ; a new separate bony sheath is 

 formed on the notochord at that part, in front of that which encloses the pith of the 

 vertebra; and behind the cranial part this has already, in this larva, coalesced with the 

 former. 



This bony rod has the unossified lateral rudiments (fig. 4, ?'y') attached to its narrow 

 fore part ; they will be ossified from it afterwards ; and thus the " odontoid " rudiment 

 will become much broader. 



The cartilage of the skull-floor has grown in front of the large cephalostyle (est) ; 

 and a new tract of cartilage, now already confluent with the hinder or parachordal 

 part of the trabeculse, fills up the space as far as to the edge of the foramen 

 magnum. 



Through this unossified tract (tr, iv) the notochord (est) runs ; the foremost larger 

 half of it is invested with the foremost bony sheath, the apex of which in the retracted 

 condition of the notochord lies behind the Jlat " postclinoid " region, in which there is 

 no " posterior basicranial fontanelle." Thus a vertebral rudiment breaks out in the 

 fore part of the basioccipital region, a region which is related to the seventh (and 

 eighth), ninth, and tenth nerves in these Ichthyopsida, and therefore not comparable 

 to one vertebral (spinal) segment, but to a regional series of such segments. 



The basicranial floor (fig. 3) is occupied by the large fontanelle from the post- 

 clinoidal to the ethmoidal regions ; it is shaped like a long egg, with the broad end 

 foremost ; its enclosure is formed entirely by the trabeculse. 



The auditory capsules (au) are more oval and more oblique ; they are turned outwards 

 in front, and are now marked more evidently by the arched canals and bulbs of the 

 membranous labyrinth within. 



They have to be studied with the hinder skull ; for they are floored by the para- 

 cliordal cartilage, which runs outwards on each side as far as to the fenestra ovalis 

 and stapes (st), and, creeping round under the fore part of the capsules, appear in 

 front on the upper face of each capsule close behind the ascending processes of the 

 suspensoria (fig. 2, a. p). 



