MONTICULIPOEID.E HETEROTRYPID^E. 27 
Orbignyella magnopora n. sp. 
PI. X, figs. 9-12. 
This species resembles 0. expansa in mode of growth, consisting of an explanate expan- 
sion composed of one or more layers of zocecia. It is distinguished even with the naked 
eye by the unusually large, thin-walled zocecia, of which there are 3 to 4 of the intermacu- 
lar zooecia in 2 mm., while 0. expansa, although a large-celled species, has thicker walls 
and 6 zooecia in the same space. The surface is smooth and the macular are sometimes 
detected with difficulty, the zocecia being usually quite uniform in size. Walls thin; 
acanthopores and mesopores wanting. Diaphragms curved and developed at intervals 
averaging their own diameter apart. 
The large zooecia will distinguish this from all other species of the genus, as well as from 
associated bryozoa. 
Occurrence. The types are from Grimsby, Ontario, but (he species lets also been found 
at Lockport and Rochester, N. Y. 
Catalogue numbers, 35537, 35538, 35539, U. S. National Museum. 
Genus MESOTRYPA Ulrich. 
Species of this genus are readily recognized by the discoid, generally free zoarium, with 
the zocecial tubes crossed by curved diaphragms, and mesop rej bundantly developed 
and closely tabulated. The following is the only species known from strata younger than 
the Ordovician: 
Mesotrypa nummtformis (Hall). 
I 'I. X, figs. 1-4. 
Callopora nummiformis Hall, Nat. Hist. New York, Pal. II, 1852 p. 148, pi. 40, figs. 5, a, b. 
CoUoporella? nummiformis Ulrich, Geol. Survey Illinois, VIII, 1890, p. 416. 
Diplotrii/Hi milleri Ulrich, Jour. Cin< innati Soc. Nat. Hist., V, 1882, p. 245, pi. n, figs. 2-2c. 
tyfesotrypa milleri Nickles and Bassler, Hull. U. S. Geol. Survey No. 173, 1900, p. 312. 
Zoarium discoid, about 10 mm. in diameter and 3 mm. thick; base slightly concave, 
covered with a, concentrically wrinkled epitheca; celluliferous side convex, smooth, with 
maculae composed of larger zocecia and more numerous mesopores distributed at intervals 
of usually 3 mm., but not conspicuous. Zocecial apertures, according to the number of 
mesopores, polygonal or subcircular in outline; to 7 in 2 mm. .Zocecial tubes crossed by 
curved diaphragms, irregular in distribution, varying from one to four in a tube diameter. 
Mesopores closely tabulated, numerous in the younger part of the zoarium, and often clos- 
ing up as the surface is approached. Acanthopores apparently not present. 
A comparison of the type specimens of the Rochester shale form, Callopora num. mi- 
form is Hall, with those of Mesotrypa milleri (Ulrich) from the corresponding horizon at 
Osgood, Ind., shows that both are founded upon the same species. Among the associated 
forms, Orbignyella expansa (Ringueberg) has curved diaphragms, but the explanate growth, 
absence of mesopores, and larger zocecia readily distinguish it. The only associated 
discoid species, Oeramopora imbricata Hall, is otherwise so distinct that there is little dan- 
ger of confusing the two. 
Occurrence. — Rather uncommon in the Rochester shale at Lockport, Rochester, and other 
localities in New York, and at Grimsby and Thorold, Ontario. More abundant in the 
Osgood beds at Osgood, Ind., where Ulrich 's types were obtained. 
Catalogue numbers, 35542, 35543, 35544, U. S. National Museum. 
Family HETEROTRYPID^] TJlricli. 
Genus STIGMATELLA Ulrich and Bassler. 
Hitherto this genus has been supposed to be represented only in the Ordovician, and 
therefore the discovery of a Silurian species is not without interest. The zoarium is 
extremely variable, ranging from incrusting to massive and ramose, although each particu- 
lar method of growth is constant for a species. The particular generic character, however, 
Bull. 292—06 3 
