1867.] BOYD DAWKIXS — EHIXOCEEOS LEPTOKHINUS. 223 



is a most accurate figure of the leptorhine premolar 2, fig. 137 re- 

 presents the first and second milk-molars, instead of the first and 

 second premolars (Pm. 2 and 3) of the tichorhine Ehinoceros. In 

 my essay on the latter species full evidence is given for this conclu- 

 sion. In the lower jaw, figured by Professor Owen, from Clacton, 

 the symphysis extends as far back as the middle of premolar 3. 



A remarkably fijie lower jaw from Lexden, containing the entire 

 permanent set of teeth, with the exception of premolar 2, belongs to 

 the same individual as the upper teeth from that locality described 

 above. 



The brickfields of Ilford, which have fiu-nished the most perfect 

 head of the leptorhine Ehinoceros, have yielded also numerous lower 

 jaws belonging to the same species. One, in the possession of Mr. 

 Antonio Brad}^, F.G.S., consisting of both rami, shows the spatulate 

 termination of the jaw. On the rectangular area, anterior to the 

 first premolars (premolars 2), formed by the horizontally flattened 

 symphysial portions of the rami, are small depressions on the outer 

 border, which probably are traces of embryonic incisors and canines. 

 .Several remarkably fijie lower rami from the same locality are also 

 in the collection of Dr. Cotton, P.G.S. 



6. Dental Formula. — Although we have no absolute evidence as 

 to the number of the milk-teeth of i?. lej>torhiaiis, the fact that in 

 all the cases in which the milk- dentition of the genus Bliinoceros 

 has been examined, it consists invariably of four teeth on either side 

 of both jaws, leaves no room to doubt that this extinct species also 

 possessed the same dental formula : ^^. 



An examination of the entire dental series of the upper and 

 lower jaws derived from the brick-earths of Lexden and Ilford j)rove 

 that, like the tichorhine and Etruscan species, the permanent dental 

 formula of the leptorhine was 



I. CO Pm. 2. 3. 4 M. 1. 2. 3 

 I. Co Pm. 2. 3. 4 M. 1. 2. 3* 



7. Measurements. — The measurements taken at the base of the 

 crown, in inches and tenths, are uniform with those of the preceding 

 essays on the tichorhine and megarhine dentition. They are : — 



1. Antero-posterior, taken along the outside of the crown. 



2. Antero-transverse, taken across the anterior lobe of the tooth. 



3. Postero -transverse, taken across the posterior lobe of the tooth. 

 A comparison of the Tables of the leptorhine and megarhine 



measiu'ements proves that the teeth of the former are, oq the whole, 

 smaller than those of the latter. 



