CAMBRO-SILURIAN MlCRO-PAL^ONTOLOGY. 33 



Batostoma Manitobense. (N. Sp.) 



Plate 9, figs. 3-3c. 



Zoarium irregularly ramose, branching at unequal intervals, diameter 

 of branches varying between six and twelve mm. Surface nearly 

 even, but exhibiting at intervals of about five mm. more or less con- 

 spicuous clusters of lai-ge cells. Zocecia with broad-oval or sub-poly- 

 gonal apertures, averaging three-tenths of n millimetre in diameter ; 

 those in the clusters attaining a diameter of half a millimetre. Over 

 the spaces between the clusters the apertures of the zooecia are ranged 

 in moderately regular series, with about thirteen in five mm. When 

 in a good state of preservation these apertures are surrounded by an 

 exceedingl3^thin peristome, upon which an occasional very small acan- 

 thopore may be detected. The interspaces in this condition are 

 slightly depressed with the mouths of the mesopores closed. As usual 

 in species of this genus the number of mesopores varies greatly in 

 different specimens. When they are the most abundant, then the 

 apertures of the zocjecia are also the most rounded. When, on the 

 other hand, the latter are more nearly polygonal, the mesopores are 

 correspondingly few, and care must often be exercised or they may be 

 overlooked. 



In tangential sections, the minute structure appears to conform in 

 every respect with the requirements of the genus, save that the acan- 

 thopores are unusuall}^ small. Just beneath the surlace the mesopores are 

 often nearly filled by deposits of sclerenchyma on the outer side of the 

 zooecial walls. In the average section, however, the ring-like walls of 

 the zocecia are rather thin, though much thicker than the partitions 

 between contiguous mesopores. In some sections the acanthopores are 

 veiy few, in others, one or two to each zooecium. 



Vertical sections show that in the axial region diaphi-agms are only 

 developed at remote intervals, or they may be absent entirely, and that 

 the tube walls are very thin, and ofcen a little wavy. The tubes bend 

 outward from the beginning and gradually becomedirectto the surface. 

 As they approach the surface their walls become much thicker, and a 

 greater oi- less number of mesopores and four or five or more dia- 

 phragms are developed. The latter are mostly less than a half tube 

 diameter apart. In the mesopores six or seven diaphragms occur in 

 a space equal to three-tenths of a millimetre. 



Compared with other species, £. Manitobense is separated by the 

 greater average number of mesopores, rounded apertures of the zooecia, 

 and the unusually inconspicuous acanthopores. B. Jamesi and B. impli- 

 catum (both Nicholson, sp.) have thicker walls and much larger and 



