12 



Membranipora Normani n. sp. (PI. XXII, figs. 5 a— 5 c) is also a partial exception 

 to the above rule, as the obliquely ascending frontal part of the terminal wall 

 can be split into two, which on the other hand does not seem to be the case 

 with the horizontal part of this wall. 



Of the two other walls we may first consider the frontal (or oral), which 

 presents the most numerous modifications and is therefore systematically the most 

 important, hi a preliminary paper' I have proposed to divide the cheilostom- 

 alous Brijozoa into four groups: Malacostega, Acanthostega, Coilestega and Camaro- 

 stega, which are to a larger or smaller degree based on the structure of this 

 wall. Of these the first three correspond with the division instituted in this work 

 under the name Anasca, which covers all the cheilostomatous Bryozoa with no 

 compensation sac, while the fourth corresponds to the Ascophora provided with 

 such a sac. Though I only intend to keep two of these names for systematic 

 divisions, it would be practical to use adjectives corresponding to all the four 

 names, in order through them to indicate essential differences in the structure of 

 the frontal wall. This may namely be represented only by a membrane (mala- 

 costegous Cheilostomatd) in a larger or smaller part of its extent, or is quite cal- 

 cified (stereostegous Ch.). In the latter case the chalk cover may be arched (ca- 

 marostegous Ch), or it may be depressed and encircled by projecting margins (coilo- 

 stegous) Ch.). Finally, above the covering membrane there may be a chalk cover 

 consisting of two rows of hollow spines connected with each other in different 

 ways (acanthostegous Ch.), and lastly, we may just recall that the calcareous skel- 

 eton itself may either be a Gymnocyst, a Cryptocyst or a compound of both. 



Before we try to give a view over the appearance and extension of the Gym- 

 nocyst and the Cryptocyst within the dilferent families of the cheilostomatous 

 Bryozoa, we may discuss some criteria, which might help to determine the pres- 

 ence of these two kinds of skeleton ih cases where there is no possibility of 

 deciding the question directly, namely, by observation of the membrane which 

 should always cover the Cryptocyst. This applies not only in most cases to the dried 

 Bryozoa, but the thin membrane is also torn away from many colonies pre- 

 served in spirit, and I may mention as an instance, that I have had to examine 

 many spirit specimens of Escharoides Jacksoni before finding the covering mem- 

 brane. The lateral and oral spines always, as mentioned before, spring from the 

 border of a Gymnocyst, and therefore we can with certainty take it for granted 

 that every calcification which appears within such a spine-bearing border or 

 from a corresponding border in a non-spinous species is a Cryptocyst. Harmer^ 



' 56, p. 2; ^ 19, p. 326. 



