434 SYSTEMATIC PALEONTOLOGY 



spines, usually six or seven in 5.0 mm., rarely five or six in the same 

 space. Spines usually about their own diameter (0.4 to 0.5 mm.) in 

 height, and nearly the same distance apart; when perfect often showing 

 a small apical pit; when worn two or three may show. Interspaces flat 

 or slightly concave, covered with minute, often confluent, granules or 

 spines, generally arranged in numerous radiating rows on the slopes of 

 the large spines. Between these granules the mouths of numerous small 

 tubes half again as large as the granules surrounding them, may be seen, 

 but it is difficult to detect them except in worn examples or those slightly 

 etched with acid. Finally the surface may exhibit, but never as a con- 

 spicuous feature, delicate branching grooves— astrorhiza — traversing the 

 middle of the spaces between the large spines. 



A rough vertical fracture shows that the skeleton is traversed by 

 numerous approximately vertical but more or less irregular tubules. The 

 skeleton shows further an irregular lamination, due to the presence of 

 interlaminar spaces or chambers occupying the spaces between the large 

 spines and placed in roughly horizontal series. Small spines, represent- 

 ing the small superficial spines of previous layers, project from the lower 

 floor of these chambers. The inner part of the skeleton, representing 

 the portion that has replaced the shell of the gastropod, is not chambered, 

 but appears to be made up wholly of minute irregular vertical fibers 

 and tubules. Similar tissue, but arranged in somewhat concentric man- 

 ner, makes up the thick walls of the large surface spines; which, appar- 

 ently, are not continuous from layer to layer. 



This extremely abundant hydrozoan evidently is a true member of 

 the calcareous section of Hydractinia. When it first came to hand, I 

 regarded it as probably the same species as H. plioccena Allman, which 

 must be very near, if not specifically identical with H. circumvestiens 

 (Wood) and H. michelini Fisher. Careful comparisons with the pub- 

 lished figures and descriptions of that and other European species, how- 

 ever, have shown that the Maryland specimens are characterized by much 

 smaller and more closely arranged spines, so that I find myself some- 

 what reluctantly obliged to propose a new name for them. As a rule 

 the large spines of H. midtispinosa are but half the size, and twice as 

 numerous in a given space, as those studding the surface of H. circum- 



