BRACHIOPODA. 99 



1885. Acrofhele, Matthew. Transactions Royal Society of Canada, Section iv, p. 39. 

 1SS6. Aerothele, 'Walcott. Bulletin No. 30, United States Geological Survey, p. 107. 



Diagnosis. " Shell corneous, composed of several laminse, the inner smooth 

 and polished, the outermost one rough and opaque. Ventral valve slightly 

 conical, with excentric umbone, pierced by a minute foramen, in front of which 

 there are, at least in one species, two small wart-like protuberances ; the field 

 between the umbone and the posterior margin is usually a little flattened, thus 

 forming a slight indication of a false area. Dorsal valve with marginal um- 

 bone, consisting of two wart- like protuberances. In the interior of the dorsal 

 valve there are two oblong, diverging muscular scars, close to the posterior 

 margin, and two small rounded scars near the middle. The muscular scars are 

 separated by a longitudinal ridge."* 



Type, Aerothele coriacea, Linnarsson. 



Observations. With the interior characters of this genus and those of the 

 closely related group Acrotreta imperfectly known, the essential distinction at 

 present recognizable in the two is in the relative development of the subapical slope. 

 This feature in Acrotreta is often very conspicuous, producing strikingly conical 

 shells which have their apices truncated by the foraminal opening and the 

 posterior moiety of the surface nearly vertical. In Acrothele the foramen in 

 the pedicle-valve is also apical, but the slope from the apex to the posterior 

 margin is gentle, often scarcely defined. The small wart-like protuberances 

 lying in front of the foramen, as indicated by Linnarsson for the type-species,f 

 and also seen in the accompanying figures of A. Matthewi, Hartt (Plate III, figs. 

 25-29), have been made from the illustrative specimens used by Mr. Matthew.j: 

 Though the function of these bodies may not be understood, they appear to be 

 homologues of the apical callosity in Acrotreta (e. g., A. Baileyi, Matthew), 

 and the mammiform swelling about the foramen in Siphonotreta. In the 

 interior of the brachial valve, the most persistent feature, as fsxr as observed, is 

 the axial ridge, which is sometimes accompanied by faint diverging muscular 

 ridges on either side near the beak {A. Matthewi), dividing the halves of an obscure 

 circumbonal muscular impression. 



* LiNKARSSON. On the Brachiopoda of the Paradoxides Beds of Sweden, p. 20. 



t LiNNARSsox. On the Brachiopoda of the Paradoxides Beds of Sweden, pi. iv, tigs. 44 b, 49 b. 



X Transactions of the Royal Society of Canada, pi. v. 1885. 



