DESCRIPTIONS OF GENERA AND SPECIES. 
Class BRYOZOA Ehrenberg. 
Order CTENOSTOMATA Busk. 
The zoarium in this relatively small order is horny or membranaceous, and consists of 
tubular stolons, from the internodes of which the zooecia are developed. The zocecia are 
usually isolated and have a terminal orifice which in the living state is closed by an oper- 
culum composed of setae. 
When compared with the other orders of bryozoa, species of Ctenostomata are not com- 
mon in the Rochester shale, five representing four genera having thus far been found 
This, however, is not unusual, as the order is poorly represented in all the geologic forma- 
tions. The infrequency of fossil Ctenostomata may be accounted for by the prevailing 
character of their zoaria, the zoarium in the majority of the living types of this order being 
membranaceous, and hence incapable of preservation in the fossil state. 
One of the Rochester shale species, Rhopalonaria attenuata, is represented by numerous 
specimens; another, Vinetta radiciformis, is uncommon; while the remaining three, Vinella 
multiradiata, Ascodictyon siluriense, and Allonema waldronense are comparatively rare. 
Continued searching will probably reveal the presence of species belonging to another 
ctenostomatous genus, namely Heteronema, since representatives of this genus are known in 
other Silurian strata. 
Mr. Ulrich and the writer have recently made the fossil Ctenostomata the subject of a small 
monograph «, and the following descriptions are based mainly on this work. Since this 
publication, no new species have been found in the Rochester shale, although several new 
forms have come to light in other horizons. 
JTamily- RHOPALOISTARIID^] HNTicltles and. Bassler. 
Genus RHOPALONARIA Ulrich. 
All the species of this genus closely resemble each other, since, as a rule, nothing remains 
but the impressions of the stolons — clay-filled or empty excavations in the body incrusted. 
However, experience has shown that the variation in the dimensions of these more or less 
accurate impressions serves as well in discriminating the species as the zoaria themselves. 
Rhopalonaria is distinguished from all other genera of the Ctenostomata by the fusiform 
internodes or cells, by their pinnate arrangement, and by the fact that they excavate their 
host. 
Rhopalonaria attenuata Ulrich and Bassler. 
PI. IV, figs. 4,5. 
Rhopalonaria attenuata Ulrich and Bassler, Smithsonian Miscell. Coll. (Quart, issue), XLV, 1904, p. 268, 
pi. 66, figs. 4, 5. 
In all the specimens at hand this species is represented by a series of excavations upon the 
surface of the object it incrusted, crinoid columns, cystid plates, brachiopods, and bryozoans 
being used indiscriminately. These impressions show that the zoarium consisted either 
of slender stolons constricted at rather regular intervals or of segments slightly fusiform in 
a Smithsonian Miscell. Coll. (Quart, issue), XLV, 1904, pp. 256-294. 
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