282 SIDNEY F. HARMER. 



operculum are large, and ori<i;inate from the lateral pnrts of 

 the basal wall, close to the distal end of the zooccinm. The 

 divaricator muscles originate from the lateral walls of the 

 zooecium, in the same neighbourhood. 



The zoarium is bounded by a tubular cavity, which is, 

 however, not continuous, as in the preceding species. The 

 cavity is a direct prolongation of one of the longitudinal 

 lines of zooecia, and may be compared with what Smitt calls 

 a ^' Samknopp " (1865, p. 6). After extending a length of 

 perhaps five ordinary zooecia it ceases abruptly, and a new 

 marginal tube is formed as a prolongation of the series of 

 zooecia next internal to it. The lateral tubes are connected 

 with the adjoining zooecia by communication-pores. 



The ovicell (fig. 10) of this species is well known from 

 Busk's description. It is very prominent, and is provided 

 with a median longitudinal keel, on each side of which is a 

 large elongated fenestra (/.). This is simply a membranous 

 deficiency in the outer wall of the ovicell, which is composed 

 of two calcareous layers. The fertile zooecium is distin- 

 guished from the others by a peculiarity in its calcareous 

 frontal wall. Instead of having a single pair of prominences 

 on the proximal side of the operculum, it has several such 

 prominences (figs. 16, 17), the arrangement of which is some- 

 what variable. There are either several tubular calcareous 

 bars radiating towards a point in the middle of the zooecium 

 (fig. 16), or a short series of bars disposed more definitely in 

 pairs (fig. 17). The latter arrangement seems to occur 

 typically in those fertile zooecia which have an ovicell on 

 their proximal side, and the former in a fertile zooecium 

 which is the first of a longitudinal series. 



The resemblance of these bars to the frontal bars ^ of a 



' I suggest the term " frontal nicnibianc " for the membranous body-wall 

 wliicli is siretched over tiie "aperture" of a Membraiiipora (fig. 43). 

 The *' frontal bars," or " costules " of some authors, are the bars which grow 

 over tills membrane in a Cribrilina (figs. 8, 44) ; wiiile I propose the term 

 " frontal sliield " for tiie calcareous part of tlie frontal surface of aCiicilostome. 

 The frontal shield is proi)ably not homologous in ail Clieiloslomes, but tiie 

 term may be used as a purely descriptive one, 



