174 
PALEONTOLOGY OF NEW YORK. 
A. marginalis* * * § the cone is broad, obtuse at the apex and the anterior curves of 
the ribbon are not materially extended- 
In young individuals the cones appear to be broad, low and obtuse, and the 
ribbon makes but few volutions. The form and structure of the brachidium 
was represented in a series of beautiful figures, by Mr. R. P. Whitfield, in 1868,f 
and some of these were reproduced in the Fourth Volume of the Palaeontology 
of New York. The peculiar structure of the loop as a pair of separate pro¬ 
cesses, was first accurately figured by Qdenstedt,^ and afterwards described 
and illustrated by Mr. William Gurley. § The character of these lamellae 
has been given in the diagnosis of the genus, but it is highly probable that 
these lateral processes of the loop were not discrete in all stages of growth. 
Mature specimens frequently have the extremities of the process in so 
close apposition that to all appearances they are united; young individuals 
rarely show any trace of disunion at the center of the loop and often no 
evidence of unusual thickening at this point. Mr. Davidson, who has called 
attention to the interrupted loop in A. reticularis, also figured in the same 
work II a preparation of A. margirialis in which the loop is continuous. A 
specimen of A. marginalis in which the lateral processes of the loop are distinct 
is figured on Plate LIV, fig. 24. 
After examination of a considerable number of preparations of the loop 
made from immature specimens, it seems highly probable that this process was 
disrupted as the age of the individual and the strain upon the loop from the 
rapid growth of the spiral coils increased. Should this proposition be sup¬ 
ported by more detailed investigation, it will help to an explanation of the 
uninterrupted condition of the loop in all stages of growth in the atrypoid 
genera, Zygospira, Glassia, Atrypina, etc. They are forms which virtually 
antedated the appearance of Atrypa, and the more elementary condition of 
* Davidson has shown that the spii’al ribbon in this form is fimbriated, and this chai-acter we also find 
well preserved in natural preparations of the spirals of A. refic-iiZam fi'om the Hamilton formation of Clarke 
county, Indiana. 
t Twentieth Ann. Kept. N. Y. State Cab. Nat. Hist., pp. 141-144, pi. i, figs. 1-8. 18157. 
I Petrefactenkunde Deutschlands, Brachiopoden, pi. xlii, figs. 87a, 90. 1871. 
§ Proceedings American Philosophical Society, vol. xvii, p. 337, id. xiv. 1878. 
I British Silurian Brachiopoda, Suppl. p. 113. 1882. 
