BRYOZOA. 231 
Homotrypella (?) ovata.) 
in3mm. Apertures oblique in some of the young examples, nearly or quite direct 
in the others; in the latter the numerous small acanthopores cause more or less 
irregularity in the outline of the apertures. Mesopores of unequal sizes, irregular in 
arrangement, scarcely more numerous than the zocecia, from which it is sometimes 
difficult to distinguish some of the larger ones. 
Internal characters: These are but illy preserved in the two sets of sections 
prepared, and all the characters shown in them are brought out in figs. 10, 11 and 
12, on plate XXVI. Four vertical sections fail to exhibit any positive evidence of 
either diaphragms or cystiphragms, the tubes appearing as open throughout. This 
condition, however, seems unnatural and probably due to imperfect preservation. 
There should be some transverse partitions in the tubes, though these, especially 
the cystiphragms, must have been comparatively few in this species. A similar 
absence of diaphragms, in this case obviously due to imperfection, is sometimes met 
with in sections of H. gracilis (Chetetes gracilis Nicholson), of the Hudson River rocks, 
which the present species is believed to resemble more than any other. And yet I 
am satisfied that, when sufficiently good material can be studied, the internal char- 
acters will prove equally as near to those of the associated H. instabilis, with which I 
had at first confounded it. 
Formation and locality.—Rather rare in the middle third of the Trenton shales, at Minneapolis and 
St. Paul, Minnesota. 
HomotryPELta (?) OVATA, 2. Sp. 
PLATE XVIII, FIGS. 23-30. 
Zoarium small, ramose, branches generally compressed, sometimes subcylin- 
drical, varying between 2 and 5 mm. in diameter or width, dividing at unequal inter- 
vals. Surface without monticules, but exhibiting at intervals of 2 or 3 mm. clusters 
of cells of larger size and more widely separated than the average. Zocecial apertures 
rounded, commonly a little oblique, oval and enclosed by a thin but slightly elevated 
peristome on which a single small acanthopore is in most cases to be detected, 
though generally with some difficulty. Interspaces depressed, with the mouths of 
the rather large mesopores occupying them, closed or open, probably according 
to the state of preservation. The zoccial rims are nearly always in contact with 
each other at limited points, yet many individual zocecia, especially of those in the 
clusters mentioned, may be completely separated from their neighbors by mesopores. 
In some specimens, preserved unusually well, the interspaces are granulose, the 
granules seeming to form rows on the walls separating the mesopores. Long diam- 
eter of average zocecium 0.17 mm.; some of the largest in the clusters 0.25 to 0.30 
mm.; ten to twelve in 3 mm. 
