ATHYRIS. 149 



1877. Spibigeea Eoissti, Gosselet. Ann. Soc. Geol. Nord, vol. iv, p. 313. 

 1896. Athteis ? concexteica and bfgulosa, Whidborne. Proc. Geol. Assoc, 



vol. xix, p. 375. 



Description. — Shell, in the young state, transverse, flattisb, with the ventral 

 valve flattest, without fold or sinus. Ventral valve with small, deep adductor 

 scars, and diffuse diductors. Dorsal valve with a central septum, thickened 

 posteriorly, supporting a broad recurved hinge-plate, which seems to intrude 

 iuto the beak, and the anterior corners of which support the short crura. Muscle- 

 scars paired, each pair being elongate and narrow, while a small internal 

 visceral foramen is seen just in front of the adductors. Spires consisting of 

 about nine whorls, and filling the whole cavity of the shell. Surface covered 

 with very numerous concentric ridges, which are often paired, and appeal- 

 generally smooth, but sometimes show signs of spiniferous elongations. 



Shell, in the older state, becoming large, and sometimes very transverse, with 

 a narrow median septum and strong dental plates. Fold broad, flatly oval, 

 protruding in front. Surface covered by extremely numerous lamellar stria?, 

 which become more crowded and definitely spiniferous near the margins. 



Size. — Large distorted fragments measure 30 mm. 



Localities. — Very common in most localities, e. g. Upcott Arch, Croyde Bay, 

 Ashhill Quarry, Braunton Down, Incheldon, Pouch Bridge, Roborough, Poleshill, 

 South Petherwyn. 



Specimens figured by Davidson as Terebratula elongata, Schlotheim ? and as 

 Athyris concentrica, von Buch ?, are in the Museum of Practical Geology ; and 

 others by Sowerby, as Atrypa hispida and A. clecussata, are in the Woodwardian 

 Museum. 



Remarks. — Two forms, both of which are not uncommon in the Pilton Beds, 

 appear to be the young and old states of the same species, and are united by 

 some intermediate but less distinct examples. 



(1) We have, first, small, flat, transverse casts, which are wide-spread and 

 common (PI. XVIII, figs. 1 — 3), and have been figured by Davidson as " Athyris, 

 sp. ? supposed to belong to A. concentrica." In these many internal details may 

 be seen. In one specimen (PI. XVII, fig. 15) the beginnings of the crura are 

 preserved. The muscular scars and the spires are occasionally shown, and as the 

 casts often occur in their moulds we can learn also the surface-ornament. This 

 ornament generally seems to consist of smooth concentric ridges, but in some of 

 the specimens these ridges are seen to be spiniferous, exactly corresponding with 

 A. hispida, Sowerby, from South Petherwyn, so that there is no doubt that that 

 species is identical with the present. A. clecussata, Sowerby, is another South 

 Petherwyn shell, which Sowerby also quotes from Barnstaple, and as his figure 

 shows a similar ornament, it is doubtless only another example of the same 



