COAL-MEASURE STEGOCEPHALIAX. 961 



presacral vertebrae, 16 in number, have an elongate hour glasss- 

 sliaped " centrum " and elongated neural arches with low elongate 

 spines. There are two processes for the i-ib, the upper carried by 

 the arch, the lower by the " centrum." There are hypRpophysial 

 in addition to the ordinary zygapophysial articulations. The 

 caudal vertebrae have no rib articulations and expanded haemal 

 arches fused into the " centium.'* 



The vei'tebral column of Geraterpeton is not well known, but 

 the Newcastle specimen of C. reticulalum seems to show the 

 following characters: — There are about 18 presacral vertebi'se 

 with hour-glass-shaped centra, long neural arches and spines 

 almost certainly with hypapophysial in addition to the ordinary 

 articulations, and a single process neaily in the centre of tlie 

 vertebra for the rib; this may be partly carried by t];e arch. 

 The later caudals differ in having no rib articulation and in 

 having large expanded hsemapophysial spines fused onto the 

 centrum. 



The vertebral columns of the two types are, in fact, veiy 

 similai', diffei^ing appai'ently only in the fact that in Diplocaulus 

 the head of the rib is double, whereas in Geraterpeton reticulatum 

 it is single. 



The shortness of the neural and haemal spines of Dvplocauhi,s 

 may be due merely to the flatness of that animal. 



The lower jaw of Diplocaulus as described by v. Huene agrees 

 very well with that of BatracMderpeton, the inner I'ow of teeth 

 at the front being probably borne on tlie real coronoid, and 

 v. Huene's complementare being an epicoronoid. 



In fact, the only important difference between the two types 

 lies in the palate. Diplocaulu,s diffei-s in its palate from Batrachi- 

 derpeton exactly as does Eryop)S from Loxomma. 



The dentitions in the two types are identical in general lines. 

 The differences a,re that in Batrachiderpeton there are large 

 pterygoids meeting in the middle line and supported by the 

 basisphenoid, in Diplocauhis there are large intei'pterygoid 

 vacuities and the pterygoids are suppoi'ted by the parasphenoid. 



I have already endeavoured to show that in the large Stego- 

 cephalia this change actually takes place, a view which if cori'ect 

 removes all difficulty of regarding Batrachiderpeton and Diplo- 

 caulus as very closely allied. 



Finally, it may be noticed that the characters in which 

 Diplocaulus differs from Batrachiderpeton are exactly those in 

 which the latter differs from Geraterpeton :— 



1. Increase in relative size of the cornua, involving larger 



squamosals, postparietals, and tabulares. 



2. More anterior position of the orbits (and their dorsal 



position). 



3. Complete loss of the nasals. 



4. Broadening of the clavicles and interclavicles. 



