72 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



in the collection of the American Museum of Natural Histor}^, proves 

 to belong to a vaHd species of Protocrisina, differing from the genotype 

 in its more luxuriant growth of rigid, closely placed, remotely bifur- 

 cating branches spread upon a plane. The zooecial structure is very 

 similar in the two species and, although the close relationship of 

 Protocrisina exigua Ulrich and P. perantiqua (Hall) is recognized, it is 

 believed that the difference in growth is sufficient for specific recog- 



/ 





Fig. 17.— Peotockisina exigtta. a to e. Wiman's views of ' i t^int i o^jlensis. a, fragment, 



NATURAL size; 6, CELLXJLIFEROUS SURFACE, XIO, SHOWING THE CIRCULAR ZOCECIA AND ACCESSOET PORES; 

 C, NONCELLULIFEROUS FACE, XIO; d AND €, TWO TRANSVERSE SECTIONS, XIO. BoRKHOLM DRIFT, OjLE 

 MYR, ISLAND OF GOTHLAND. / TO i, ULRICH'S VIEWS OF PROTOCRISINA EXIGUA. /, TWO FRAGMENTS, 

 NATURAL size; g, CELLULIFEROUS FACE, XlS, WITH ZOCECIAL OPENINGS AND ACCESSORY PORES; h, NON- 

 CELLULIFfeROUS SIDE, X18, WITH PORES; i, TRANSVERSE SECTION, X18. FeRNVALE LIMESTONE OF RICH- 

 MOND GROUP, Wilmington, Illinois. 



nition. This same close relationship between Black River or early 

 Trenton and Richmond species has been noted in a number of 

 instances. Three well-marked new species are known. The first is 

 described in the present work as Protocrisina ulricM, while the sec- 

 ond and third, which await description, are not uncommon in the 

 Nematopora bed of the Trenton in Minnesota, and in the Middle 

 Ordovician (Ottosee) shales of eastern Tennessee, respectively. 



