252 Mr. T. Davidson's Notes and Descriptions 



septum is likewise perceptible, the muscular impressions being 

 also much alike in this valve and only slightly modified in dif- 

 ferent species : — see our figures of T. psittacea, T. octoplicata, T. 

 sub-Wilsoni, PL XIII. fig. 1, 2 & 8. Prof. M'Coy in 1844 

 noticed the form of these short lamellae in Ter. acuminata, where 

 I have seen them, but is mistaken when stating that he believes 

 them to be the same in T. hast at a ; in this species they form a 

 loop as in true Terebratulce, as may be seen from the perfect 

 specimen now preserved in the museum of the Geol. Survey; 

 we must therefore conclude that there exists no important dif- 

 ference in the internal organization of the imperforated valve. 



Much stress has been laid on the form of the beak, foramen, 

 and deltidium in separating these shells into distinct genera ; let 

 us therefore consider the true value of these characters in some 

 of the forms alluded to. 



In all, the beak is acute and not truncated. In R. psittacea 

 (type of his genus Hemithiris), M. D'Orbigny asserts the foramen 

 to be without any deltidium ; this is however a mistake : the 

 deltidium is there j rudimentary, but existing, and partially cir- 

 cumscribing the foramen. In Rhynchonella, he mentions the 

 foramen to be entirely surrounded by the deltidium, which is 

 stated to be tubular (PI. XIII. fig. 3) ; but this also is far from 

 being the case in many species admitted and placed by that 

 author in Rhynchonella, such as R. concinna (PI. XIII. fig. 6), 

 &c, wherein not only the deltidium is not tubular, but it does 

 not even surround the foramen, a portion of which is completed 

 by the umbo of the smaller valve*. Again, in many species, such 

 as R. tetrahedra (PI. XIII. fig. 7), serrata, spinosa, &c, the beak 

 becomes in age so much recurved as to exhibit no visible fora- 

 men or deltidium, or passage for the muscular fibres of attach- 

 ment, so that the shell probably became free, a circumstance well 

 known to be peculiar to some extinct genera of Brachiopoda. It 

 is therefore evident that the distinctions sought for cannot be 

 found in either the beak, foramen or deltidium, in the shells com- 

 posing these three genera proposed. 



If we now examine the impressions left in the interior of the 

 perforated valve, we shall find that these vary in some species 

 placed in the same genus by M. D'Orbigny ; that they are not 

 peculiar to any of the types, or constant in any of the species 

 forming the different genera : thus if we cast a glance on the 

 interior of this valve of R. psittacea (Hemithiris) and many 

 others believed to be Rhynchonella, we shall find no perceptible 



* Mr. Woodward called my attention to some very young specimens of 

 JR. concinna (Ter. Flabellulum, Sow.), in which the deltidium exists only 

 jn the same rudimentary form as in the adult of the recent R. psittacea. 



