M .. ,. . BEYOZOA. 219 



Montloullpora Incompta.] 



MONTICULIPORA INCOMPTA, n. Sp. 

 PLATE XV, FIGS, 0-12. 



Zoarium parasitic, forming inconsiderable crusts or small masses upon shells, 1 or 

 2 cm. in diameter and several mm. thick. Surface even, with inconspicuous clusters 

 of cells of slightly larger size than the average. Zooecia polygonal, with thin walls 

 (comparatively thick for the genus), the ordinary size varying between 0.2 and 0.25 

 mm, in diameter, while those in the clusters mentioned do not exceed 0.32 mm.; 

 about eight in 2 mm. True mesopores wanting, the small cells being shown by thin 

 sections to be merely young zocecia. Acanthopores small, inconspicuous externally. 



The internal structure is so well illustrated on plate XV that a detailed descrip- 

 tion is rendered superfluous. The chief peculiarity of the species is the minutely 

 cellulose and roughish character of the zooecial walls. In all other respects the 

 species agrees closely enough with M. wetherbyi and M. lamellosa Ulrich, the latter 

 from the Hudson River rocks of Illinois. 



Formation and locality. — Eather rare in the middle third of the Trenton shales, at Minneapolis, 

 Minnesota. 



Mus. Reg. No. 5968. 



MONTICULIPOKA GEANDIS Ulrich. 

 PLATE XV, FIGS. 1-6. 



Monticulipora grandis Ulbich, 1886. Fourteenth Ann. Rep. Geol. and Nat. Hist. Surv. Minn., p. 78. 



Zoarium irregularly massive, often tending to become lobate or subramose" 

 Zooecia polygonal, with thin walls. Surface without monticules, but exhibiting at 

 intervals of about 5 mm. conspicuous groups of cells larger than the average ; of the 

 latter nine or ten occur in 3 mm, while some of those in the clusters have been 

 observed to reach a diameter of 0.5 mm. No mesopores. Acanthopores very small, 

 few, practically wanting. When perfectly preserved (see plate XV, fig. 4) the cysti- 

 phragms are often to be seen in the mouths of the zooecia, giving them the false 

 appearance of being very small and situated at the bottom of a wide sloping area. 



Internal structure: In the vertical sections the zooecial tubes proceed to the sur- 

 face in straight or curved lines, according to the form of the zoarium. They are 

 provided with thin walls and usually two more or less closely arranged series of 

 cystiphragms, one on each side of the tube, the narrow intervening space being 

 crossed by an approximately equal number of straight diaphragms. The variation 

 and relative disposition of parts is shown in fig. 6 of the plate cited. The lower half 



