TREPOSTOMATA. 37 
Family AMPLEXOPORID^E TTlricli. 
Genus RHOMBOTRYPA Ulrich and Bassier. 
The quadrate shape of the zooecia in the axial region is especially characteristic of this 
genus. This feature is best observed at the ends of the branches. 
Rhombotrypa spinulifera n. sp. 
PI. X, figs. 13-16; PI. XXV, fig. 21. 
Zoarium small, ramose; branches slender, cylindrical, about 3 mm. in diameter, dividing 
at intervals of 10 mm? or more; surface smooth, macula 1 not a conspicuous feature, and 
composed of mesopores and of zooecia slightly larger than usual. In mature specimens i he 
zooecia at the surface are thin walled, rounded to elongate polygonal in outline and separated 
from each other, sometimes entirely but usually only laterally, by mesopores. Mature 
specimens also show small acanthopores rather regularly distributed and about one to each 
zooecium. In young specimens the zooecia are rhomboidal in shape and arranged regularly 
in quincunx lines. 
Tangential sections show that the mesopores are quite variable in number and shape and 
that the spines of the surface are of the nature of I rue acanthopores; 6 to 7 zooecia in 2 mm., 
measuring longitudinally, and transversely 10 to 11 rows in the same space. Transverse 
sections show the zooecia to be quadrate in outline in the axial region, 11 to 12 in 2 mm. 
In the axial region of vertical sections the zooeeial walls are thin and become but slightly 
thickened in the peripheral; diaphragms wanting entirely in the zooecia and very few in 
the mesopores. Vertical sections also indicate that new tubes are interpolated at approxi- 
mately the same level. Vertical fractures also bring out this fact by showing alternate 
zones of smooth sides and of rough edges, the zooeeial tubes exhibiting in the first instance 
the flat sides of their walls, and in the latter case exposing their angles. In order to accom- 
modate new zooecia this turning about of the tubes in the axial region becomes necessary 
because of their quadrate shape. These zones of turnings are concentric and occur at 
intervals of about 0.7 mm. 
By a glance at the quadrate zooecia exposed at the end of a branch one distinguishes this 
neat species from associated small ramose forms. The presence of acanthopores separates 
it from other species of the genus. 
Hitherto this type of monticuliporoids has been supposed to have been confined to the 
Richmond formation, the type of the genus, R. quadrata (Rominger) being a very charac- 
teristic fossil of these rocks. The discovery of the species just described in younger rocks 
is therefore of considerable interest. 
Occurrence.- - -Rather rare in the Rochester shale at Lockport, where the type specimen 
was found, and at Thorold, Ontario, but not uncommon at Rochester, N. Y. The species 
has also been noted in comparative abundance in the Osgood beds at Osgood, Ind. 
Catalogue numbers, 35594, 35595, IT. S. National Museum. 
Family CONSTELLARIID^ TJlricli. 
Genus NICHOLSONELLA Ulrich. 
The following two species are apparently the last representatives of this genus, which is 
identified for the first time in the Silurian system. As might be expected, both differ in 
some respects from the typical Ordovician species, but the general aspect of each is such 
that Nicholsonella seems the only genus to which they can be referred with some certainty. 
Both develop the peculiar character of Nicholsonella, namely, that with age the walls of 
the mesopores become obscured by a calcareous deposit filling the interzooecial spaces. 
Indeed, the walls of the zooecia and mesopores have an indefinite structure throughout the 
mature region that is in marked contrast with the clear-cut walls of Lioclema, to which 
genus both species had formerly been referred. 
