MancJiestcr Memoirs, Vol. Iv. (191 1), No. II. 3 



line of descent, but in proving the continuance of this 

 type of ornament in Liassic times. 



In the Inferior Oolite occur two types with concentri- 

 cally marked umbones, showing them to be derived from 

 the pure concentric M. curvirostris type. 



{a) T. v-costata. In these the area is pure con- 

 centric, as is the flank in nepionic stages. In later stages, 

 however, these concentric flank markings break up near 

 the marginal carina into alternating tubercles which 

 rejoin diagonally, forming a marked V in that region of 

 the shell. For the formation of at least one V one pair of 

 anterior costre unite to join one posterior limb. This is 

 important, as it is a constant character throughout all forms 

 which have V's or V-like undulations, and it is retained 

 after the disappearance of this undulation. 



{b) T. costatula. Here the ornament is purely con- 

 centric until late ephebic and gerontic stages where the 

 flank markings begin to break up near the marginal 

 carina into alternating tubercles. 



Of these two types, T. v-costata seems to have reached 

 a definitely phylephebic stage of ornament, and T. 

 costatula to be still in a phylo-nepionic state. 



The Great Oolite shows the continuation of T. 

 costatula in T. painii. Here the nepionic stages are 

 M. cjirvirostris and T. costatula. The ephebic show the 

 introduction of further alternate pustulation, and the 

 elongation of these pustules to form marked undulations 

 near the marginal carina, though not the formation of a V 

 as in T. v-costata. 



Here also occurs T. clytia which is a descendant of 

 T. v-costata, differing from it in the regular junction of two 

 anterior costse to one V after nepionic stages. 



In the Forest Marble is found T. undulata which 

 appears to be a further evolution of T. painii. In the 



