J. W. Jackson—Brachiopod Morphology. 25 
Brachiopods of the Terra Nova Expedition, is not by any means peculiar 
to the Rhynchonellids. It is present in Liothyrina, Terebratulina, 
and in all the short-looped forms I have been able to examine, some 
twenty-four recent species. It is also present in several fossil 
forms, including the large Crag fossil Zerebratula grandis, T. bisinuata 
from the London Clay, and Cyclothyris latissima from the Lower 
Greensand of Farringdon. In Terebratulina cancellata, Koch, the 
pedicle collar is strongly developed, and forms a perfect tube extending 
forward for some little distance, whilst in Megathyris decollata, Chem., 
it is well-developed and is supported by a sharp mesial septum. It is 
a structure which grows larger as the shell increases in size, and 
is the ‘‘doublure sous-apicale’’.and ‘‘doublure sous-cardinale’’ of 
Fischer & Oehlert.? 
Judging from my own collection the pedicle collar is never 
developed in the higher long-looped forms; it is entirely absent 
from the following genera: Jlagellania, Terebratella, Dallina, Macan- 
drewia, Terebratalia, Laqueus, and Frenulina (type sanguinea). In 
some of these, however, there is occasionally a thickening in the 
umbo around the foramen assimilating a pedicle collar, but it is 
fused to the shell and never free anteriorly.’ 
_In the lower genera referred to the family Terebratellide a curious 
feature is present: Jlihlfeldtia truncata and Megerlina lamarckiana 
both possess a pedicle collar, but Araussina rubra does not; it only 
has a thickened plate fused to the floor of the umbonal cavity. 
Genus rHerA,’ Thomson. 
With regard to the relation of Rhynchonella lucida, Gould, to 
Thomson’s new genus thea, I possess a specimen of this form and 
find that it possesses fairly strong dental plates. This fact, together 
with the hypothyrid character of the pedicle passage, excludes the 
species from Atheia. Like Hemithyris psittacea there is no true 
cardinal process in the dorsal valve, the divaricator muscles being 
attached to the posterior ends of the crural bases. 
_ The absence or presence of a cardinal process in Rhynchonellids 
seems to be a matter upon which some difference of opinion exists. 
In H. psittacea I have failed to find any structure iba ees to a true 
cardinal process, but in two large adult specimens of ‘ Hemithyris’ 
nigricans in my collection from Chatham Islands there is a small but 
distinct transverse bilobed cardinal process extending outwards like 
a shelf from the apex of the valve; the mesial septum in these 
specimens is also much stronger than in H. psittacea. In the ventral 
valve the dental plates are recurved towards the apex and net 
vertical as in H. pszttacea. 
Regarding dental plates, etc., a close study of the fossil fering of 
the nigricans group would no doubt reveal some interesting features. 
The Manchester Museum possesses three specimens belonging to this 
group from the Table Cape Beds at Wynyard, Tasmania. One of 
these specimens is apparently closely related to H. nigricans; the 
1 Hxpéd. Scient., pp. 44, 108, ete. 
2 In old adult shells of Hemithyris psittacea the pedicle collar sometimes 
becomes fused to the floor of the umbonal cavity of the ventral valve. 
