Dr. J. Allan Thomson — On the Thecidiinoe. 461 



with the essexites and its intermediate texture, it may be included 

 in the essexite-monchiquites. Lacroix 1 has applied this term to 

 those monchiquites in which felspar predominates over nephelite 

 and which have, in addition, an isotropic (presumably analcitic) 

 groundmass. The Madagascar rocks differ from the one under 

 consideration only in the presence of barkevikite and in a greater 

 scarcity of olivine. The name is appropriate in the case of the 

 Craighead rock, since, by an increase in the grain-size of the minerals, 

 a gradual passage may be traced to a normal essexite. 



Another ' spotted ' rock represents a more felspathic and finer- 

 grained type than the one described above. The phenocrysts are not 

 so abundant, and the olivine rivals the augite in size. As before, 

 both these minerals are poecilitically enclosed in felspar, which, 

 however, also occurs as laths, of varying dimensions and often 

 arranged in a radial fashion. The phenocrysts, together with 

 a second generation of augite and numerous prisms of apatite, are 

 enclosed in a matrix which is mainly analcite. In both of these 

 rocks, local patches of a more acid nature occur. Olivine and 

 porphyritic augite are absent, and the ' clots ' consist of phenocrysts 

 of labradorite in a groundmass of analcite, with subordinate nephelite 

 and granular augite. These seem to be of late formation and to arise 

 through the crystallization of a residual magma rich in water and 

 alkalis. 



(To be concluded in our next Number.) 



VII. — On a New Genus and Species of the Thecidiin^: 

 (Beachiopoda). 



By J. Allan Thomson, M.A., D.Sc, F.G.S. 



THE subfamilies Thecidiinae, Dall, and Leptodinae, Schuchert, 

 constitute the family Thecidiidae, Gray, which is regarded by 

 Schuchert as a near relation of the Strophomenidae. The chief 

 external characters of the Thecidiinae are the smallness of the shells, 

 the absence of the foramen, attachment by the ventral valve, the 

 presence of a nearly straight hinge-line and of a prominent area with 

 a solid deltidium. The shell substance, with the exception of the 

 deltidium, is punctate. Internally the ventral valve bears in its 

 hollow beak a small median septum on which is sometimes superposed 

 a small muscular plate. The dorsal valve bears a so-called cardinal 

 process, formed by the median union of the socket ridges, and this 

 plate is strong, subrectangular, and hollow at its base, and projects 

 beyond the hinge. In most of the genera two lateral spurs unite 

 mesially to form a bridge just in front of the cardinal process, over 

 the visceral cavity. There are no free brachial arms, but the brachial 

 supports are represented by an anterior septum, frequently branched, 

 and lamellae rising from the floor of the valve in the spaces between 

 the septum and its branches, the margins, and the bridge. The 

 septum runs back from the anterior margin towards the bridge, and 

 like the margins and the bridge, is more or less covered with 

 granulations. 



1 Nouvelles Archives du Museum [4], i, p. 142, 1903. 



