8 March, Morphogenesis of certain Pclecypoda. 



Affinities of the UndnlatcB \^— Undulated {Lye.) and 



Gibbosce, Lyc\ 

 The closest connection of the Undulatie appears to 

 be with the Costatit [Lycett]. It has been said that no 

 Costate Trigonia has ever shown the faintest trace of 

 concentric ornament on the area. There is, however, in 

 the Manchester Mnseum a T. denticnlata [Inf. Oo.] with 

 pustulations on the marginal carina which agree in 

 position and number with the ends of the flank costai. 

 Similarly arranged tubercles occur on the inner carina, 

 and joining these two are regularly arranged rows of 

 tubercles. A very young specimen from the Bradford 

 clay, possibly T. detiticniata, hints at the same arrange- 

 ment. 



From the Rluctic, of Bristol, comes iMyophoria 

 eninnichi, which has purely concentric flank markings and 

 predominant radial markings on the area with a trace of 

 concentric. This can only be seen in a few specimens, 

 and even then with difficulty, as they are not well 



preserved. 



In the l^rias occms M. lineata {Fig. II. oi Plate), which, 

 as described before, has pure concentric flank markings 

 and predominant concentric markings on the area, 

 showing the introduction of radial near the inner carina. 



This traces the costate type of ornament back into 

 Triassic times. Its later development takes place in the 

 Tertiary epoch. The £'^f^?/^ Australasian form, T.subnn- 

 dnlata, shows pure concentric marking on the flank, except 

 close to the marginal carina, where it becomes interrupted, 

 showing the introduction of radial ornament. 



The Miocene and recent Trigoniae show tuberculate 

 radial ornament over the whole flank, except for a small 

 space below the umbo, near to the anterior edge of the 

 shell. Here occur concentric lines, which at the back, meet 



