90 Canon A. M. Norman — Notes on the 



(? rosette-plates)^ the appearance of which was exactly that 

 of those figured as characteristic of the genns St eq an op or a 

 (see d^Orbigny, Paleont. Francj., Terr. Cret. pi. dccxxi. figs. 3, 

 7,11; and Jullien, " Les Costnlides/' Bull. Soc. Zool. de 

 France, 1886, pi. xvii. fig. 2 and j)l. xix. fig. 2). 



The Zocecium-building in Cribriliuidse. 



In describing the structure of Lepralia {MemhraniporeUa) 

 nitida, Hincks wrote that " in its earlier condition it closely 

 resembles one of the spiniferous Membranipora. The ribs of 

 the adult state are represented by suberect spines set round 

 the margin. As growth proceeds the spines bend inwards 

 and increase in size, and gradually take on the flattened rib- 

 like appearance. After a time the opposite rows meet in the 

 centre of the cell-area, and the extremities, Avhich are often 

 enlarged, are soldered together, so as to form a well-marked 

 median line. In some instances the ribs unite laterally to a 

 great extent, and the front becomes a solid wall. In others 

 they continue quite separate." In his account of Cribrilina 

 annulata he writes : — " In its earliest stage the zooecium 

 exhibits the simple Membraniporidan form — the area occu- 

 pying the entire front and being closed by a membranous 

 covering. The ribs which compose the frontal Avail in the 

 adult are given off on each side as tubular processes from the 

 edge of the nascent cell, and, gradually lengthening, meet in 

 the centre and unite, the line of junction giviug the subcariuate 

 appearance of the zooecium. These tubular girders, which 

 are probably the equivalent of the marginal spines on many 

 of the Membramporo! , are connected at intervals by lateral 

 outgrowths of calcareous matter ; and in this way the porous 

 structure of the furrows is produced. The mode of formation 

 may be well studied in the thickened anterior margin of the 

 orifice, which is composed of two tubular pieces, the pointed 

 extremities of which in meeting often bend outwards and 

 give rise to the central mucro. Sometimes they are not 

 closely Avelded together, but overlap one another or remain 

 partially separate, so as to give a bifid appearance to the 

 mucro. Occasionally they do not unite at all, but continue 

 permanently free and detached." 



All this is in every particular correct as regards the 

 two species referred to ; and the description of Cribrilina 

 annulata J of course, was intended to be applicable in a general 

 sense to the Avhole of the species which he placed iu Gray^s 

 genus Cribrilina. My object in the folloAving notes is to 

 go intothe question more muiutely, for the purpose of showing 



