16 On some Teeth of new Chimceroid Fishes. 



posterior measurement of the largest specimen (41866) being 

 only 0*015 m. 



Formation and Locality. — Kimmeridge Clay, Weymouth. 



Genus Elasmodectes, Newton. 

 [Mem. Geol. Survey, iv. 1878, p. 43 (Elastnor/nathus) .] 



Elasmodectes secans, sp. n. (PI. III. fig. 3.) 



Diagnosis. — Mandibular tooth with a moderately sinuous 

 oral margin, and the post-oral border inclining backwards 

 more than the symphysial border ; outer tritors very small 

 and undivided, coarsely laminated. 



Remarks. — The type specimen is the small left mandibular 

 tooth shown of the natural size in PL III. fig. 3, from the 

 inner (a) and outer aspect {h). The beak is unfortunately 

 broken away, revealing the small beak-tritor in section ; but 

 the tooth is in other respects complete. It is remarkable as 

 being the most sectorial form of mandibular dentition hitherto 

 discovered in Jurassic rocks. Though differing from the 

 typical Cretaceous Elasmodectes in the simple character of the 

 outer tritors, it does not appear advisable on present evidence 

 to refer the tooth to a distinct genus ; for the normally simple 

 outer mandibular tritors of Ischyodus are partially subdivided 

 in some species, and such subdivision ought not thus to be 

 always noted in generic diagnoses. When the palatine teeth 

 of E. Willetti and of the form now described are discovei-ed, 

 it may be possible to determine definitely whether the two 

 fishes in question are generically identical. 



Formation and Locality. — Kimmeridge Clay, Weymouth. 



EXPLANATION OF PLATE III. 



Fig. 1. Pachymylus Leedsi, gen. et sp. nov. ; pair of palatine teeth, oral 

 aspect, two thirds nat. size. 1 a. Right palatine tooth, outer 

 aspect, two thirds nat. size. 1 b. Left palatine tooth, inner 

 aspect, two thirds nat. size. — Oxford Clay, Peterborough. 

 [Leeds Collection.] 



Fig. 2. Ditto ; right mandibular tooth, inner aspect, two thirds nat. size. 

 2 a. View of symphysis of same specimen, two thirds nat. size. — 

 Ibid. 



Fig. 3. Elasmodectes secans, sp. n. ; left mandibular tooth, inner (a) and 

 outer (6) aspects, nat. size. — Kimmeridge Clay, Weymouth. 

 [Brit. Mus. no, 43284.] 



