278 MYRIOZOID.E. 



Mastiyophora Hyndmanni the lateral sinuses are in some 

 states very apparent, and in others almost disappear. 

 In all cases this peculiarity in the form of the orifice is 

 due to the degree in which it is constricted near the hinge 

 of the operculum, and is a mere matter of detail, and not 

 a change of type. In S. venusta the variation seems more 

 striking, from the orbicular form of the orifice above and 

 the great width of the sinus. I do not think that we can 

 isolate this species from the present genus. 



Lepralia obvia, described by Manzoni from the Pliocene 

 of Castrocaro, agrees with S. venusta in the form of the 

 mouth. 



Genus MASTIGOPHOEA, Hincks. 



Der. From ^d<m, a whip, and <f>epu, to bear. 



FLUSTHA (part.), Audouin. 

 LEPRALIA (part.), auctt. 

 I IK RENT i A (part.), Gray. 

 HIPPOTHOA (part.), Smitt. 



GENERIC CHARACTER. ZoraciA with a semicircular ori- 

 fice, the inferior margin straight, with a central sinus ; 

 furnished with lateral vibracula. ZOARIUM (in British 

 species) incrusting. 



THE presence of fully developed vibracula in place of 

 the more usual avicularian appendages distinguishes the 

 present genus from the preceding. The character seems 

 to be one of considerable importance, both morphologically 

 and physiologically, and may fairly be used, I think, to 

 mark off a separate group. 



