312 DR. A. VAUGHAN ON - THE PAUNAL SUCCESSION [May I906, 



Athyris has very strong, slightly-diverging dental plates, and the 

 radiating ridges on the cast resemble low ribs. 



Athyris glabristria, A. expansa, and A. planosulcata. 



I have recorded Athyris glabristria from various levels throughout 

 the Eush sequence. 



Athyris expansa is common in the Megastoma-~Beds, and a single 

 specimen is recorded from the Cyathaxonia-Beds. 



Our knowledge of Athyris is, unfortunately, very incomplete, 

 owing to the rarity with which specimens exhibit unweathered 

 expansions. The distinction between a sheet-expansion — built up 

 of tubular ribs webbed together, as in Actinoconchus, or the group of 

 Athyris planosulcata, Phil., and a fringed expansion — in which the 

 free margin of the expansion is composed of completely-detached, 

 flattened, tubular spines, as in Oliothyris, or the group of Athyris 

 pectinifera (Sow.), has been found to be of little assistance in the 

 determination of specimens such as are usually met with in strati- 

 graphical work. Specimens showing expansions are only to be found 

 on weathered surfaces, and the continued weathering of a sheet- 

 expansion removes the web faster than the thickened rib, thus 

 producing a fringed margin. Unless I have been extremely un- 

 fortunate, this is the common case with specimens of Athyris plano- 

 sulcata. The fringed Athyrids from the South- Western Province 

 have been very carefully examined by Mr. T. P. Sibly and myself, 

 and we have arrived at the definite conclusion that, with the probable 

 exception of A. cf. pectinifera from D 2 of Gower, the detached 

 spines never extend to the suture-line of the expansion, but they 

 always spring from a more or less narrow basal lamella. Again, 

 one part of the shell may appear to have fringed expansions, whereas 

 the expansions in another part may be lamellae, without any sign 

 of fringe. 



On the other hand, Seminula is usually separated with ease, by 

 the absence of any trace of expansion and the terebratuliform lines 

 of growth. 



For these reasons, 'Athyris' is here employed to 

 cover all those Athyrids the expansions of which are 

 either actually fringed, or may become so by wea- 

 thering. It follows that ' species '-distinction becomes little 

 more than form-denotation ; the species-groups thus made are, 

 therefore, in the nature of ' circuli,' and are, from a stratigraphical 

 point of view, of little value. 



The terms are here employed in the following sense : — 

 Athyris = Athyrids which exhibit fringes (either original or 

 produced by weathering). 



A. glabristria, Phil., has a gibbous, transverse form, with a 

 broad mesial swelling and shallow depression ; the beak is thick, 

 with rounded slopes. The accentuation of the mesial swelling into 

 a strong fold, standing out from the flanks, is, apparently, in the 

 South- Western Province, distinctive of the Z 2 forms. 



A. expansa, Phil., has a flattened, very transverse form with 

 almost uniplanar valve-intersection and a non-prominent beak. 



