44 FISH OF THE OLD RED SANDSTONE. 



deflected edge; external border convex_, much raised and 

 strongly undulato-plicate, the ridges being largest in front, 

 and gradually diminishing towards the external and posterior 

 portion of the tooth ; the plicse are produced by a thickening 

 of the substance of the ridges and a scooping-out of the inter- 

 vening hollows, so that the under side of the tooth remains 

 even ; under surface coarsely osseous ; upper surface polished, 

 with small obscure undulations and minutely porous. 



This genus is closely allied to Ceratodus and Ctenodus, but 

 differs in the grinding surface being concave, the tooth resem- 

 bling the inside of a plicated oyster. The internal microscopic 

 structure was developed for me by the kindness and skilful 

 manipulation of my friend Mr. Anthony of Caius College, Cam- 

 bridge ; it was very complex and peculiar, but the prepared frag- 

 ment has unfortunately been mislaid, so that I am unable now 

 fully to describe it. I only know one species, the following. 



Conchodus ostreaformis (M^Coy). 



About 1^ inch long, 1 inch wide, and 1| line thick, the 

 grinding surface deeply concave, the surface of attachment equally 

 convex; the external semicircular margin gives origin to six or 

 seven coarse, rugged, converging ridges, the most anterior about 

 7 lines long and slightly inclined to the straight inner mar- 

 gin, the most posterior is about 2 lines long and at right angles 

 wdth the inner margin ; the ridges are separated by deep, wide 

 hollows. 



Found along with Dendrodus latus (Ow.), Holop. giganteus 

 (Ag.), and Hoi. princeps (M^Coy), in the old red conglomerate of 

 Scat Craig. 



(Col. University of Cambridge.) 



