256 PROC. COTTESWOLD CLUB vol. xill. (4) 



10. MiCROTHYRIS TARDECRESCENS, 5. Buckman, sp.n. 



PI. XIII., fig. la, b, c. 



Description: — A somewhat elongate, globose shell, with nearly 

 equi-convex valves, which are marked with close-set, distinct ridges 

 of growth. Front margin obtuse, slight!}' indented. Beak short, 

 curved over umbo. Beak ridges verj' short, only defined just under 

 foramen, the beak appearing to have no ridges ; foramen small. 



Comparison: — There is much similarity to Terebr. 

 imitator, except for the growth ridges. A TerebrattUa 

 figured by Deslongschamps as Terebratula sphceroidalis, 

 Brachiopodes jurassiques. Pi. Ixxxi., fig. I, is similar in 

 shape, and also has growth ridges, which are, in fact, 

 exaggerated growth lines. 



Distinction : — From Zeilleria fei'ruginea, the difference 

 in shape, but particularly the absence of well-defined beak 

 ridges. 



Remarks : — The aspect of the beak is almost that of a 

 Terebratula ; but there is a long and fairly marked dark 

 line indicative of a septum in the brachial valve. The 

 beak is of quite a different character from that of the 

 Z.-Waltoni series with the sharp beak ridges, or from the 

 incurved beak of siibcormita, which also has distinct beak 

 ridges. The character of the beak seems to justify the 

 placing of this species to Microtkyris, E. Desk, whereof 

 lagenalis is the type. At any rate it does not deserve to 

 be called Zeilleria. 



Localities and Strata: — Dorset: Burton Bradstock, from 

 the upper beds ; Bradford Abbas, from about the horizon 

 of the Marl bed. Somerset : Stoford. A scarce shell, but 

 most distinctive, and several specimens have been obtained. 



Date of Existence : — Garantiancs hemera. 



Note : — The character of ridges really arises from exag- 

 geration of valve-thickening, alternating with normal growth 

 to finally become a settled feature. The ridges presumably 

 mark times of slow growth. 



