164 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



mm., transversely six to seven rows occur in the same distance. Inter- 

 zooecial spaces as wide as the apertures and having a carina which 

 carries numerous small spines or acanthopores. Sloping areas about 

 the apertures of variable shape, but usually polygonal with a ten- 

 dency to hexagonal. 



All of the features believed to be seen in thin sections are shown in 

 figure 82 h to e. Both transverse and vertical sections show that 

 the immature region is composed of thin-walled zooecia with a few 

 complete diaphragms. In the mature zone the walls thicken greatly 

 and the usual cryptostomatous cell is developed. Tangential sec- 

 tions indicate the oval zooecia with thick interspaces in which rows of 

 small pores indicate the course of the surface caringe. The maculse 

 are composed entirely of such interzooecial tissue pierced by pores 

 as shown in fig. 82 c. 



Occurrence. — Somewhat uncommon in the Jewe limestone (Dl), 

 Baron Toll's estate, Esthonia. 



Holotype.—Csit. No. 57256, U.S.N.M. 



One specimen and a thin section of the type are in the collections of 

 the British Museum. 



NEMATOTRYPA, new genus. 



Zoarium of slender, solid branches composed of long, tubular 

 zooecia diverging obliquely from a central, thread-like axis. Surface 

 smooth with oval zocecia arranged in rather regular, longitudinal 

 rows. Interzooecial space filled by numerous small mesopores open 

 at the surface. Hemisepta and acanthopores or spines as in other 

 members of the family. 



Genotype. — Nematotrypa gracilis, new species. JMiddle Ordovician 

 of Russia and Sweden. 



The name which has been chosen for this genus has reference to 

 the thread-like central axis from which the zocecia proceed to open 

 at the surface as so many perforations or apertures. The name will 

 also in part recall Nemataxis, another genus of the Rhabdomeson- 

 tidse, which is believed to be related. The internal arrangement of 

 the zooecia is much the same in the two genera, but in Nemataxis the 

 interspaces are solid instead of being occupied by open mesopores 

 as in Nematotrypa. 



The species upon which Nematotrypa is based is a very abundant 

 fossU in several of the Middle Ordovician formations of Baltic Russia 

 and Sweden. Fragments from the Lycldiolm limestone of the island 

 of Dago, too small for the recognition and description of another 

 form, probably represent another species. 



