Bd. III: 7. 
ANTARCTIC FOSSIL BRACHIOPODA. 
1 1 
Hemithyris imbricata, sp. n. 
PI. I, fig. 12. 
Desa-iption {ventral valve only)-. Broadly pentagonal, ornamented with numerous, 
somewhat stout, rounded radial costae, which are crossed by growth lines somewhat 
conspicuously; and where the crossings occur there is imbrication — the test of the 
rib being raised into an incipiently spinous projection. 
The ribs increase in number by intercalation and by bifurcation, at irregular 
distances from the beak. The new rib so produced is of smaller size at first than 
the older ones, so that there is some irregularity of ribbing. 
There is a distinct mesial sinus. 
Affinities: The nearest species appears to be Rhynchonella doederleini, DAVID- 
SON (Rec. Brach. 172 PI. XXV, fig. 14, 15) a recent shell from Japan. That is in 
the truly spinous stage; while this Antarctic form is in the incipient spinous stage: 
another step in development would convert its projecting imbrications into hollow 
spines. 
Of R. doederleini Davidson says: ‘When young the ribs are few in number, 
and the spines have not yet been formed.’ The statement means that young R. dœ- 
dcrlcini goes through a stage comparable to that of the adult of the present species; 
thus the young of that species is the morphic representation of the adult of this, or 
the adult of this one is the morphic préfiguration of the young of that; though the 
two species may not stand actually in the relation of ancestor and descendant. 
From R. nigricans the imbrications distinguish this form well, and the ribs are 
more approximate. 
Remarks-. This specimen appears to possess no dental plates, and as it is evi- 
dently near to R. nigricans, a species of Hemithyris, this would seem to be its 
correct generic designation. R. doederleini has been regarded as a species of 
Acanthothyris on account of possessing spines; and that genus is said to extend 
from Jurassic to Recent. (SCHUCHERT, 325; et auctt.) But spinosity is in itself 
not a generic character, it is only a stage of development to which various stocks 
attain. R. doederleini can have no connection with the Jurassic species of Acantho- 
thyris; and it is more probably a spinous development of Hemithyris nigricans: the 
Antarctic form is in the intermediate stage betwen the two. 
Locality. Cockburn Island (13), off Graham Land, Antarctica. 
Formation'. Glauconitic Bank. 
Material: One imperfect ventral valve. 
