432 BRITISH PALALOZOIC FOSSILS. [Bracuropopa. 
Genus. ATHYRIS. See page 196*. 
ATHYRIS AMBIGUA (Som. Sp.) 
Ref. and Syn. = Spirifer ambiguus Sow. Min. Con. t. 376.= Terebratula id. Phill. Geol. York. Vol. II. t. 11. 
f. 21. = Atrypa sublobata Portk. Geol. Rep. t. 38.f. 2; = Terebratula ambigua de Kon. Anim. Foss. Belg. 
t. 20. f. 2.=id. id. M. V. K. Geol. Russ. t. 9. f. 12. 
Desc.—General form varying from elongate-ovate to rotundato-subtrigonal, and transversely pentagonal ; 
edges of all the forms varying from acute to very obtuse (in rare very old examples) ; lateral margins very rarely 
in one plane (in the ovate varieties), more usually (in the pentagonal varieties) with a very deep rounded sinus 
towards the entering valve in the middle, with sometimes nearly as strong a sinus on each side towards the re- 
ceiving valve; generally a minute notch in the middle of the front margin. Entering valve varying from ovate to 
pentagonal, of moderate but variable convexity, evenly convex near the beaks and posterior sides, and either evenly 
convex also in the anterior portion, or a very broad quadrate mesial ridge, left by the abrupt bending downwards 
of the lateral margins into a sinus on each side; in nearly all specimens a deep, narrow, more or less defined, 
mesial sulcus extends from the notch in the front margin nearly to the beak. Receiving valve moderately convex ; 
greatest depth about the middle of its length ; with a deep, angular, mesial sulcus usually extending nearly or quite 
to the beak, widening towards the front margin; generally defined by an obscure angularity on each side, ex- 
tending the width of the broad mesial sinus in front; beak moderate, obtuse, with a rounded perforation in the 
apex; spiral appendages very large. Surface smooth, rarely with a slight radiating lineation, minutely fibrous 
under a strong lens. Width of pentagonal examples about eleven lines; proportional length of receiving valve 
80 5 
varying from *, to #4, length of entering valve =, depth of both valves varying from = to = (the latter very 
100 
13 35 
rare,) width of sinus in front margin =, depth thereof varying from ;;, to rarely =; length of ovate specimens 
100 50 
about eight lines, width varying from “ to more than *”, depth of both valves varying from 7; to =. 
This species, as will be seen from the above description, is extremely variable: the original figures in the 
Mineral Conchology represent two of the most common pentagonal forms, the larger unusually flattened, the two 
upper figures more near the average of ordinary specimens. In the Isle of Man limestone the pentagonal 
typical forms are common, with specimens gradually assuming a greater depth and obtuseness of edge than 
I have seen elsewhere, and the lateral and front sinus in the margins very strongly pronounced. 
In the Derbyshire limestone the rather flattened, pentagonal, wide forms, with searcely any lateral sinus in 
the margins, and a very wide, moderately elevated front sinus, are not uncommon, gradually passing into the 
ovate forms, in which the length is sometimes greater than the width, and in which even the sinus in the front 
margin is faintly marked, although usually notched, and giving rise to a narrow mesial sulcus in each valve. I 
have at length succeeded in tracing, in the most gradual manner, the passage of all the forms figured by 
Col. Portlock, under the name of Atrypa sublobata, into each other, and into the ordinary types of the present 
species. When decorticated a few straight pallial ridges are seen near the beak, radiating towards the front 
margin. Col. Portlock notices the resemblance of some of the varieties to the S. wnguiculus of Sowerby, but 
the want of area between the beaks and hinge-line separates the species. The ends of the fibres give a very 
minutely granular appearance to the surface under a strong lens, but I think there is no true punctation. 
Position and Locality—vV ery common in the lower carboniferous limestone of Derbyshire; very common 
in the lower carboniferous limestone of Isle of Man; not uncommon in the carboniferous limestone of Flint- 
shire; strongly marked specimens of small size common in the lower carboniferous limestone of Lowick, 
Northumberland; in the carboniferous limestone and shale of Beith, Ayrshire; common of small size in the 
carboniferous limestone of Berwick-on-T weed. 
* Isee that by some accident, in the character given at the above page, this genus is stated to be without foramen, 
from my old diagnosis, which escaped observation in correcting the proofs. I formerly supposed, with Professor Phillips, 
that the minute opening at the apex of the beak of the receiving valve was caused by fracture; continued observations 
haye, however, since shewn me that it is a natural character of the genus as the continental authors contended. 
