18 



One of the most distinctive features of the species is seen in vertical sections. 

 The walls of the zooecia are very thin in the sub-mature region, but assume their 

 great thickness very suddenly as the mature region is reached. The dark 

 boundary line between the walls of the zooecia is very clearly seen in vertical 

 sections. In the mature region, diaphragms cross the tubes at distances, 

 approximately equal to the internal diameter of -the latter, but are much less 

 frequent in the immature region. 



Although in the opinion of the writer (see p. 22), the division of the trepo- 

 stomatous bryozoans into the Amalgamata and Integrata, on the basis of the 

 presence or absence of the dark line bounding adjacent zooecial walls, is an 

 artificial one, still it must be admitted that certain forms show this boundary 

 line more clearly than others and can with convenience be assembled into the 

 family Amplexoporidae. Amplexopora solitaria is one of these forms and is 

 the only member of the family with acanthopores to be found on the Credit 

 river. It can be distinguished from other species of the genus by the great 

 thickness of the zooecial walls and by the sudden manner in which they thicken 

 on passing from the immature to the mature zone. 



Locality. — -Credit river, Streetsville. 



No. 12147, Royal Ontario Museum of Paleontology. 



Atactoporella densa, sp. nov. 

 Plate I, Figures 2 and 4 



A single example of a bryozoan, showing the internal characters of the 

 genus Atactoporella, was found in the zone of Bythopora meeki in the Streetsville 

 member. It is not well preserved, but it differs so decidedly from any previously 

 described member of the genus from the Cincinnatian series that it has been 

 decided to erect a new species for its reception. 



The zoarium consists of four superimposed layers, which are rather thick 

 for the genus, each layer varying from two to three millimetres in a vertical 

 direction. It could not be ascertained, on account of the poor preservation of 

 the fossil, whether the surface is monticulose or smooth. It apparently is not 

 an-incrusting form. 



In characters shown by both tangential and vertical sections, A. densa 

 resembles A. ortoni (Nicholson), of the Maysville of Ohio. The tubes are 

 small, nine to ten in two millimetres, with the comparatively thick walls lined by 

 average-sized acanthopores. As many as twelve acanthopores have been counted 

 surrounding a single zooecial tube. The mesopores, so far as can be determined, 

 are not as numerous as in A. ortoni, and are not filled by secondary deposits 

 as in that species, but are open throughout their length. The mesopores are 

 crossed by numerous diaphragms, while the zooecia have both diaphragms and 

 cystiphragms. 



In addition to the differences already enumerated, A. densa differs from 

 A. ortoni in the manner of growth. The latter species forms exceedingly thin, 

 monticulose crusts, usually attached to the shells of brachiopods. The Streets- 

 ville form differs from A. schucherti, Ulrich, the only other member of the genus 

 found in the Richmond, in the much smaller size of the acanthopores, in the 

 smaller number of mesopores, and in the manner of growth. 



Locality. — Credit river, Streetsville. 



No. 12148, Royal Ontario Museum of Paleontology. 



