l82 NEW YORK STATE MUSEUM 



expression and the easy flexibility in the development or suppression 

 of their surface characters, may make the group a convenient 

 receptacle, even though the characters can not be very strictly 

 delimited. Hydnocerina, then, is characterized by a reticulum bear- 

 ing more or less prominent rings crested by one, two or even three 

 rows of nodes, the nodes being often of irregular size; sometimes 

 long and simple, sometimes low and duplicate, frequently fusing 

 together and thus losing their distinctness. 



HYDNOCERINA ARMSTRONGI Sp. nOV. 



(Plates 3 and 5) 



This species is represented by two specimens which present a 

 fairly complete conception of the entire reticulum, the larger being 

 almost the entire horn ; the smaller, the initial end of. a specimen 

 which might well have been of the same dimensions as the larger. 

 The aspect of the initial part of the reticulum is interesting as dis- 

 playing very clearly the development of the exterior features. 

 Prism- faces are almost wholly undeveloped, although suggested; 

 annulations appear at once and these are quite simple. This small 

 specimen carries six of these annulations, on the summit of which, 

 at the crossing of the prism-lines, are intimations of nodes. At 

 once thereafter, apparently, the nodes spring into greater promi- 

 nence and the annulations at first become more upstanding through 

 deeper constriction of the intervals. Growth appears to have been 

 regular for a matter of three or four additional annulations, the 

 nodes developing in such fashion that up to this point the species 

 would be safely placed with the genus Hydnoceras. Then com- 

 mence the irregularities which take it outside the boundaries of 

 that genus. The annulations become broader, grooved on top; the 

 nodes appear first small and then large, or small and large together, 

 some of the rings being made up of a single row of large nodes as 

 below, and other rings being broad, duplicate, or triplicate, grooved, 

 with the nodes quite low and confluent. The larger of the speci- 

 mens shows eleven annulations in which the character of the 

 nodosity may be indicated from below upward as follows : 



1 Large and simple 



2 Large, some of the nodes fused laterally 



3 Large, some of the nodes fused laterally 



4 Double row of low nodes 



