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,J and afterwards described 

 and illustrated by Mr. William Gdrley. § 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. marginalis 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 Atbypa, and the more elementary condition of 



* Davidson hasBhonn that thespii'al ribbon in this form is fimbriated, and this character wp also find 

 well preserved in natural preparations of the spirals of A. retictt/aris from 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. 18C7. 



X Petrefactenkunde Deutschlands, Brachiopoden, pi. xlii, figs. 87a, 90. 1871. 



$ Proceedings American Philosophical Society, vol. xvii, p. 837, jil. xiv. 1878. 



I British Silurian Brachiopoda, Suppl. p. 113. 1882. 



