ACROTRETID^. 635 



AcrotTiele avia Matthew, 1903, Geol. Survey Canada, Eept. Cambrian Rocks Cape Breton, pp. 98-99, PI. V, figa. 



la-f and 2a-b. (Described and discussed essentially as in Matthew, 1902a, pp. 396-398. Figs, la-f are copied 



from Matthew, 1899b, PI. Ill, figs, la-f; figs. 2a and 2b are copied from Matthew, 1902a, PL XVII, figs. 2a and 



2b, respectively.) 

 Acroihele abavia Matthew, 1903, idem, pp. 100-101, PI. IV, figs. 3a-d and 4a-b. (Described and discussed as in 



Matthew, 1902a, pp. 398-400. Pigs. 3a-d and 4a-b are copied from Matthew, 1902a, PI. XVI, figs. 3a-d and 



4a-b, respectively.) 



This is a strong shell of the Acrothele matthewi (Hartt) type. The adult shells are some- 

 what thicker, the surface is a httle different, being coarser, and the callosity in front of the 

 pedicle opening is longer and larger than in A. mattJiewi or its varieties. Matthew [1902a, 

 p. 396] gives as one of the most marked distinctions of this species the presence on the sides 

 of the dorsal valve of "about a dozen radiating, branching, crenulated ridges that extend 

 to the margin." With all of Matthew's types before me, I find one bit of rock with two ventral 

 valves, one partly overlapping the other; on these the radiating ridges not only occur on the 

 sides, but also over the middle of the shell, and where a bit of the shell is broken off the ridges 

 are clearly impressed on the cast of the interior; this specimen is labeled as one of the original 

 types of the species; a second ventral valve preserving a part of the exterior surface also shows 

 traces of radiating ridges. Another type specimen shows part of the exterior surface and 

 the cast of the interior, but there is no trace of a radiating ridge on the outer surface or cast; 

 seven specimens of the dorsal valve on which the outer layer is exfoliated show no traces of 

 the radiating ridges. I find further that the dorsal valves on which the radiating ridges occur 

 are very thin and hence liable to be flexed by lateral pressure, developing surface ridges on 

 the fines of radiating striae that occur on the inner layers of nearly all sheUs of Acrothele. On 

 the ventral valve a few radiating ridges occur on one of the specimens (PL LXI, fig. 7) ; another 

 specimen, laterally compressed, shows many such ridges (PI. LXI, fig. 7a). Other specimens 

 are without exterior traces of ridges, but when the shell is exfoliated, numerous fine, radiating 

 striaB with stronger and deeper strise at wider intervals are to be seen. I have taken up this 

 matter of the radiating ridges at length, as I do not consider the side ridges on the dorsal valve 

 ■of Acrothele avia sufficient to separate it from other species to wlfich it is closely related. 



The surface is formed of fine, irregular, concentric, rounded ridges that frequently inoscu- 

 late ; these ridges are crenulated by fine depressions which cross them and thus outUne tubercles 

 or rounded points of varying height and size, depending upon the depth of the radiating cross 

 depressions; this type of surface gives great variety to different parts of the shell and to dif- 

 ierent sheUs. As far as I can determine by the use of a strong lens (three-quarter inch) it 

 is essentially the same type of surface as that occurring on Acrothele matthewi (Hartt) and 

 related species, one of which is shown by Plate LXI, figure 4d. 



At the apex of one of the specimens of the ventral valve in Matthew's collection there 

 Are two minute elongate tubercles with a narrow depression between them, and on the pos- 

 terior slope there is an oval pedicle opening with the narrow end nearest the apex; a low, 

 narrow ridge, that divides the obscure false area midway, extends from the opening to the 

 posterior margin. 



"Acrothele abavia" Matthew does not appear to vary specifically from the forms of 

 Acrothele avia in wlfich the radiating ridges are absent. The specimens occur in a little coarser 

 sediment, and on this account are not well preserved. Matthew [1902a, p. 399] speaks of a 

 smaU tubercle in front of the pedicle opening and "the visceral callus that extends half of 

 the length of the shell" of the ventral valve; such a tubercle occurs on the only two specimens 

 of A. avia weU enough preserved to show the interior of the ventral valve. I do not find the 

 elongate visceral caUus on any specimens of the ventral valve in Matthew's collection; some 

 specimens do show a short caUosity similar to that in A. avia. After a very careful study 

 of aU the specimens, also of a large collection from the same locality and strata belonging to 

 the United States National Museum, I am led to think that Matthew nustook the median 

 ridge of some dorsal valve in which the posterior portion was broken away as the visceral 

 callosity of the interior of a ventral valve ; this befief is further strengthened by his statement 

 [1902a, p. 399]: "On each side of the callus a groove runs out toward the front margin. Some 



