62 BRYOZOAN FAUNA OF THE ROCHESTER SHALE. 
Genus DIAMESOPORA Hall. 
Diamesopora Hall, Nat. Hist. New York, Pal. II, 1852, p. 158 (not defined). 
Diamesopora Nickles and Bassler, Bull. U. S. Geol. Survey No. 173, 1900, p. 54. 
Diamesopora Grabau, Bull. New York State Mus., IX, No. 45, 1901, p. 175. 
Diamesopora Hall and Simpson, Nat. Hist. New York, Pal. VI, 1887, pp. xv, 19 (definition based on 
species of Chilotrypa). 
Diamesopora Miller, North American Geol. and Pal., 1889, p. 30. (Includes, besides the genotype, 
species of Cozloclema and Chilotrypa.) 
Diamesopora Simpson, Fourteenth Ann. Rept. State Geol. New York for the year 1894, 1897, p. 560. 
(Definition contains in part characters of both Diamesopora and Chilotrypa.) 
Diamesopora Ulrich, Geol. Survey Illinois, VIII, 1890, pp. 380, 467; Geol. and Nat. Hist. Survey Min- 
nesota, Final Rept. Ill, pt. 1, 1893, p. 330; Zittel's Textbook Pal. (Eng. ed.), 1896, p. 268 (= Coe- 
I ode ma). 
A study of thin sections of the genotype of Hall's Diamesopora, D. dichotoma, and of 
his Trematopora osculum, a related species found very abundantly in the Waldron shales in 
Indiana, showed, as indicated before, that the genus has until recently been misunderstood 
and incorrectly defined and classified. Ulrich proposed the name Ccdochma for similar, 
hollow-stemmed Ordovician species, but before defining the genus, abandoned it under the 
impression that his forms were congeneric with D. dichotoma. As the above synonymy 
shows, this author based his definition of Diamesopora more upon species of Coeloclema than 
upon the genotype. The tw T o genera are now known to be so far removed from each other 
as to be classified under different orders, Coeloclema being a member of the Ceramoporidae, 
a family of Cyclostomata, and Diamesopora having the rhomboidal primary zocecia and 
tubular vestibule characteristic of the Cryptostomata. 
Nickles and the writer (op. (it.) defined the genus as follows: "Zoarium ramose, of hollow 
stems lined internally by an epitheca; zooecia simple, hexagonal, or rhomboidal, with an 
oval orifice in the anterior half, which, with growth, forms a tubular vestibule; apertures 
with peristomes equally elevated or highest posteriorly; intervestibular spaces compact or 
horizontally laminated." 
The diagnostic characters are the mode of growth, the crvptostomatous zocecia, tubular 
vestibule, and compact intervestibular space. Lunaria are absent, but their structures are 
simulated when the peristomes are highest posteriorly. A blunt superior hemiseptum is 
often found projecting from the zocecial wall, but is seldom a conspicuous feature. 
Diamesopora dichotoma Hall. 
PI. XXI, figs. 12, 13; PI. XXIV, figs. 28-30. 
Diamesopora dichotoma Hall, Nat. Hist. New York, Pal. II, 1852, p. 158, pi. 40B, figs. 3a-d. 
Diamesopora dicho'oma Grabau, Bull. New York State Mus., No 45, 1901, p. 175, fig. 78. 
Fragments of this species are usually found as flattened branches crushed by pressure, 
but the normal zoarium is of smooth, hollow, ramose, cylindrical stems, 2 to 4 mm. in 
diameter, with the internal cavity now occupied by clayey matter. 
The largest specimen seen shows that, as a rule, the zoaria seldom branches, a long 
straight stem of 4 or 5 cm. in length usually occurring. The crust forming the hollow 
zoarium is about 0.60 mm. in diameter, the interior or basal portion lined with a transversely 
striated basal membrane — the epitheca. Maculae inconspicuous and only distinguished by 
the absence of zooecial apertures. In young specimens the zocecia are seen to be arranged 
in regularly ascending spiral lines, but the older the zoarium the more obscured becomes 
this arrangement. The zocecia in young examples have wdl-marked peristomes generally 
elevated highest anteriorly, but these also become less conspicuous with age, and in many 
specimens the peristomes are not visible at all. 
Internal characters. — Thin sections of this species are particularly instructive, since with- 
out them the crvptostomatous character of the genus would not be suspected. They show 
(1) that the primitive zocecium is rhomboidal in outline, (2) that a tubular vestibule h 
developed in the anterior portion, (3) that at the junction of the posterior wall of the prinii- 
