EABLY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 201 



The thin layer of cells is growing upon the flat side of the shell and 

 exhibits a structure that has been noted in all species growing upon 

 Hyoliihes, namely that no true zocecia are found here. This portion 

 of the zoarium is made up exclusively of small cells differing from 

 mesopores only in having thicker walls. A discussion of this feature 

 is given on page 47, so that further remarks are unnecessary at this 

 point. 



The zoarial surface in M. expressa is smooth and without distinct 

 maculae, although groups of larger zooecia are present. The apertures 

 are quite regularly hexagonal in outline, with eight in 2 mm. Meso- 

 pores practically absent at the surface, acanthopores apparently 

 wanting. 



Tangential sections passing tln'ough the zoarium near the surface 

 show the great regularity in the shape of the aperture. The cut edges 

 of the curved diaphragms likewise frequently show in such sections 

 as curved lines crossing the apertures. Deeper tangential sections 

 exhibit mesopores and less regular zooecia. 



The chief peculiarity of vertical sections has already been men- 

 tioned. The zooecial tubes are thin walled in the immature region, 

 where closely tabulated, narrow mesopores are developed in some 

 number. As the mature zone is approached, the mesopores become 

 smaller and finally disappear entirely. Here their place is taken by 

 a slight widening of the zooecia and an increase in the thickness of 

 their walls. Slightly curved diaphragms occur in the immature zone 

 at intervals averaging their own diameter. In the shorter mature 

 zone they are four or five times as numerous and are more curved, 

 in fact they are frequently arranged on top of each other in the 

 manner of true cystiphragms. 



There is no described species near enough the present form to make 

 comparison necessary. Leptotrypa Jiexagonalis from the same forma- 

 tion has the same method of growth and similarly shaped apertures, 

 but its internal characters are quite different. A glance at the 

 fractured edge of a zoarium will suffice to distinguish the few hori- 

 zontal diaphragms of the Leptotrypa from the numerous curved 

 structure of the Mesotrypa. 



Occurrence. — Rare in the Kuckers shale (C2) at Reval, Esthonia. 



Holotype.—Csit. No. 57284, U.S.N.M. 



British Museum, one specimen and thin sections. 



MESOTRYPA MILLEPORACEA, new species. 

 Text fig. 109. 



Zoarium at first incrusting and then becoming a free, lamellate 

 expansion several centimeters wide and averaging 5 mm. in thickness. 

 Celluliferous face smooth, with indistinct maculae; noncelluliferous 

 side marked with a strongly wrinkled epitheca. Zooecial apertures 



