Polyzocij Coslenierata^ and S^ong&s. 447 



the condition by which both zooecia and sessile avicularia first 

 originate. A similar depression, though without the distinct 

 raised margins, occurs in a similar position in M. Flemingi^ 

 Busk ; and the avicularium itself of course appears in M. Dume- 

 rili and M. unicornis^ Fleming. Unfortunately Mr. Hincks, 

 when describing (Hist. Brit. Mar. Pol. p. civ) the develop- 

 ment of the ooecial ectocyst, does not give that of the ooecium, 

 and I know of no other account ; hence the homologies of 

 the different parts of the ooecium can be hardly more than 

 guessed at. The radiating furrows or ridges which are seen 

 in some ooecia, and are indicated in one stage of this one, may 

 perhaps indicate that it is primitively constructed by the 

 growing together of paired trabecular similar to those which 

 probably unite to make up the zooecial cell-front in the Cliilo- 

 stomatous forms (see Hincks, I. c. p. 184, on the genus Crihril- 

 lina^ and his description of transverse strife in the primitive 

 covering of the cell of Mucronella coccinea at the second 

 developmental stage, p. civ) . The ooecium of adult Reteporm 

 has a median cleft ; and in one species this is represented by 

 an invagination from the lower edge : this is probably the 

 remains of an embryonic median cleft, common to all typical 

 Chilostoraata, and represents the ununited edges of the cell, 

 this stage corresponding to stage 2 of Hincks's history of the 

 zooecium of Mucronella coccinea. 



The median ridge or galeate process, seen here in one stage, 

 and also found in M. aurita, Hincks, Micropora coriacea^ 

 Esper, Cliorizopora [Lepralia) Brongniarti^^nskj and Stegano- 

 porella Smitti, Hincks, perhaps represents the point of union 

 of the two edges of the ooecium, strengthened by additional 

 calcareous matter. 



Membranipora craticula, Alder. 



Metnhranipora cratimla, Alder. Trans. Tyneside Nat. F, Club, iii. p. 144, 

 pi. X. fig. 3. 



Two colonies with the typical characters. The one has the 

 surface beautifully hyaline ; the other was apparently taken 

 dead, and is consequently granular and deficient in some of 

 its parts. The number of spines is, as a rule, 12 to 14. 

 The bar crossing the surface of the ooecium is well marked ; 

 and the lateral or median avicularium is also generally present 

 here though not invariably. The avicularium below the 

 cell is often very prominent, owing to the mound on which it 

 is placed ; this point serves to bring the species still nearer to 

 M. lineata, Linn., from which it is now to be distinguished 

 chiefly by the slender characters of shape and position of the 



