512 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 39. 



CORYNOTRYPA NITIDA, new species. 



This very delicate, dainty species is clearly a descendant of the 

 Ordovician type of the genus, C. delicatula (James), and of the wide- 

 spread Silurian form, C. elongata (Vine). Indeed, these three forms 

 are so similar that it is difficult if not impossible to point out striking 

 differences. Very conservative students would probably regard the 

 three as merely varieties, if not the same species, but, in view of their 

 different geologic ages, and of the fact that some differences, however 

 sUght, may be observed, it is believed that the retention of separate 

 specific names is preferable. 



The incrusting zoarium of G. nitida has so far been noted only on 

 corals, the two type-specimens growing upon Padiypora fiscTieri and 

 CystipJiyUum americanum, respectively. The zooecia are very slender 

 and much elongated, the angle of divergence being about 20°. An 



Fig. 10.— Coetnotrypa NirroA. o, portion of a zoarium, X20, growing ttpon the coral 



PACHYPORA FRONDOSA. DEVONIAN, HAMILTON GROUP, ARKONA, ONTARIO. 6, SKETCH OF 

 A ZOARIUM, X9, INCRUSTING A CXTP CORAL; C, SEVERAL Z0CECL4. OF THE SAME, X20. SAMK 

 HORIZON AT THEDFORD, ONTARIO. 



average zooecium, including the stolon, is 0.5 mm. long and 0.07 mm. 

 across at its widest part, and 6 to 7 zooecia may be counted in 3 mm. 

 The aperture is subterminal and constricted, about one-third the 

 diameter of the zooecium, and bears a low rim. 



Compared with its Silurian progenitor, C. elongata, the present form 

 is found to be decidedly smaller and more delicate and to have a less 

 rapid angle of divergence. In general shape O. delicatula is probably 

 more closely related, but here again the zooecia, although delicate, are 

 coarser in every measurement than in O. nitida. 



Occurrence. — Apparently rare in the shales of the Hamilton group 

 at Thedford and Arkona, Canada. 



Cotypes.~Cat. nos. 54180, 54181, U.S.N.M. 



I 



