260 ME. AETHUE W, WATERS OK 



wall of the ovicell is prolonged below into a kind of lamina, 

 subtruncate at its lower extremity, which extends some way into 

 the aperture. This is a character which Hincks mentioned when 

 describing his B,. prcetenuis (=-Ii. marsupiata, Sm.), and is found 

 in this group in H. cellulosa, R. atlantica, It. complanata, and to 

 a certain extent in R. Beaniana, but not in It. mediterranea, 

 It. aporosa, and B.Jlssa, in which last the front wall ends higher 

 up and is straight. In It. monilifera, var. munita and umbonata, the 

 front wall is prolonged in the same way, but has a cleft or sinus 

 at the end of the lamina (see PL VII. fig. 20) forming a squarer 

 opening. 



The fenestral avicularium is found at the angle of most f enestrse, 

 but not of all ; similar fenestral avicularia occur in a large number 

 of species of Betepora and are also found in Petralia. As a rule 

 in Betepora they occur on the dorsal surface at the angle of the 

 f enestrse, but they are sometimes on the front, as ia B. monilifera^ 

 var. munita. 



The dorsal surface has the vibices more or less longitudinal 

 and more numerous than in B. mediterranea. 



In a specimen from the North Cape the large erect avicularium 

 has a distinct beak, and at the sides there are two projecting 

 wings (PI. VI. fig. 17). The mandible has a larg lucida, which 

 Mr. Busk described as a foramina ; but as it is caused by the chitin 

 being here thinner, I have elsewhere proposed the name lucida *. 

 This lucida is figured by Busk surrounded by a second oval, as if 

 there were a thick band ; but this structure I have not found in 

 the mandible of any, and have added a figure from a specimen 

 sent to me from the North Cape, the mandibles of which are 

 similar to those from Naples, Bapallo, and Capri. 



Smitt placed B. Beaniana as " forma " of jB. cellulosa, and 

 certainly it is very difficult to separate the group. The oral 

 avicularium may be at the end of a long rostrum, as in typical 

 B. Gouchii ; it may be shorter, as in B. Beaniana, or within the 

 oral aperture, as in B. mediterranea, or absent, as in B. cellulosa. 

 Intermediate stages are found in a series of specimens, and very 

 slight changes would evolve the one from the other. Busk 

 speaks of a prominent rostrum having a minute avicularium on 

 one side at the base, but this I have not seen. 



* Ann. & Mag. Nat. Hist. ser. 5, vol. xx. p. 84, 



