190 PEOF. W. K. PAEKEE ON THE STEUCTUEE AND 



The dentary lessens again irregularly, and ends below the hind part of the articular 

 condyle {ar. c). 



Above (fig. 3, d) it has a clear, sharpish, cultrate edge, after the horny sheath has 

 been removed, and then a wide scooped tract up to the coronoid crest. 



The splenial {spl) is about three fifths the length of the fore half of the dentary, and 

 is equidistant from the ends of that region ; it lies over the Meckelian rod [ink), between 

 it and the dentary, and is a very delicate, styliform, dentigerous splint. The other 

 bone of the mandible is ectosteal ; it is wrongly called " angulare " by Professor Huxley 

 in his short but excellent description of this skull in the ' Encyclopaedia Britannica ' 

 (Art. Amphibia, p. 758); it is the superficial part of the true " articulare." 



B. The Endocranium of Siren lacertina. 



I have figured the endocranium, cleared of the investing bones, shown from above 

 (PI. XXXIX. fig. 2), in the lesser specimen ; this structure also can be seen as showing 

 through the parasphenoid below (PI. XXXVIII. fig. 6), partly above (fig. 5), and in 

 the side view (PI. XXXIX. fig. 1) ; these are from dissections of the larger specimen. 



The occipital condyles {oc. c) are large, reniform, obliquely turned inwards and for- 

 wards towards the mid line, and are separated fi-om each other by a space half their 

 own width ; their aspect is thus inwards and backwards, and it is also downwards. 



The moieties of the basal plate (PL XXXIX. fig. 2,) are united behind by a small 

 isthmus in the younger specimen, but are distinct in the larger (PI. XXXVIII. 

 fig. 4). 



In both the hinder parachordal tracts in the floor of the skull are represented by a 

 thin, sharp selvage to the auditory capsules, but they run under the capsules and 

 appear in front of them; the basal plate extends outwards to the fenestra ovalis 

 {vl, st) as a thin underlayer. A wide heart-shaped " posterior basicranial fontanelle " 

 (p. h.c.f) exists devoid of cartilage (but floored by the parasphenoid) from the foramen 

 magnum (_/'. m) to the " posterior clinoid band " formed by the fusion of the apices of 

 the trabeculse [a. tr), from which the notochord has retired (see embryonic skull of lAsso- 

 triton, PI. XL. flg. 1, tr, nc). This narrowish band is curved backwards ; and its fore 

 edge forms the hinder boundary of the great front basal fontanelle, the so-called 

 " pituitary space," which reaches to the junction of the ethmoidal and septal regions. 



Above, the superoccipital region {oc. r) is represented by a tract of cartilage shaped 

 like a quadrant ; the exoccipitals {e.o) nearly meet behind this tract, which perfects the 

 occipital arch above. 



The moieties of the endocranium are united in the adult by this upper band, by the 

 clinoid belt below, and by the intemasal (" intertrabecular ") wedge in front ; in the 

 young we get a foui'th point of conjugation, viz. the basioccipital (PI. XXXIX. 

 fig. 2). 



The hind skull, formed by union of the two pairs of basal plates (parachordals and 



