BRYOZOA FROM ZANZIBAR. 525 



inn.ndibles occur very frequently in the genus and may be called 

 Reteporidaii aviculai'ia,. There are other small avicularia with 

 solid chitinous semicircular mandibles like those near the oral 

 aperture. These have two muscular tendons, whereas the large 

 triangular ones have a single tendon, and this muscular distinc- 

 tion between the round and triangular mandibles is very frequent 

 in the Cheilostomata. 



In the monilifera group the growth of the ovicell can be well 

 followed. Looking down the peristome the opening of a large 

 pore is seen in the distal wall (PI. LXXII. fig. 1) until the ovicell 

 commences to grow (figs. 1 & 2), when the pore is more or less 

 hidden by a disk growing on a small stalk (fi.g. 2). This disk 

 becomes concave as it grows, showing this to be the commence- 

 ment of the inner wall of the ovicell (fig. 3), then in a later 

 stage part of the front is open (fig. 5), next the slit of the upper 

 part of the trifoliate stigma is seen (fig. 6), and soon the ovicell 

 is complete (fig. 7). 



More or less similar commencements of the ovicell occur in 

 other genera, and sometimes specimens show these disks when 

 no ovicells are developed. 



There is one large rosette-plate at the distal end of the 

 zooecium, and the muscles for withdrawing the polypide are 

 arranged on each side of this with a chitinous thickening to 

 which they are attached. 



The avicularia are vqyj variable in Retepora, and we must not 

 trust to them too much in determining species. 



Log. Off Cape York, 8 fath. (i^itsA;) : Red Sea ( IFaf ers). Wasin, 

 Brit. East Africa, 10 fath. (500), 20 fath. (520), collected by 

 Crossland. 



Retepora producta Busk. (PI. LXXII. figs, 9, 10.) 



Ketepora producta Busk, Zool. Chall. Exp. vol. x. pt. xxx. 

 p. 108, pl. XXV. fig. 7 (1884) ; MacGillivray, " Tert. Polyzoa of 

 Victoria," Trans. Roy. Soc. of Victoria, vol. iv. p. 115 (1895); 

 Tliornely, Records of Indian Museum, vol. i. pt. 3, no. 13, p. .193 

 (1907). 



Retepora porceUana, var. laxa MacGillivray, Prod. Nat. Hist. 

 Victoria, dec. x. p. 15,' pl. 95. fig. 6 (1885). 



The small semicircular avicularia are very much raised, which 

 is not usual in Retepora, and are crenulate, as is frequently the 

 case in the round avicularia of the genus. These avicularia do 

 not occur on the younger zocecia but are common on the others. 

 The zooecia are much smaller than those of R. porcellana MacG. 

 and the spines are ridged, but I am not sure that it is right to 

 speak of them as articulated. In the zooecia near the border 

 there are often triangular avicularia with a single beak. The 

 ovicells in the Wasin specimens are not very much raised and 

 have a distinct median slit. On the dorsal surface there are 

 long nan-ow avicularia ; the meshes are large, but " papillary 

 eminences " are not found as a frequent character. 



