323 



may be absent. The oaria consist of a calcified endoooecium and a membranous 

 ectooa'ciuni. Uni- or multiporous rosette-plates, which may sometimes be placed 

 in pore-chambers. 



Both the form of the sinus of the aperture and the rosette-plates undergo 

 consideral)le variation in the species I have hitherto been able to refer to the 

 genus Schizoporella as defined here. Thus, whilst the sinus is extremely narrow 

 in S. spongites and S. Ion()irostris, that in .S. unicornis is broadly rounded and has 

 more than a third of the whole breadth of the aperture. We have an inter- 

 mediate form in regard to the development of this sinus in S. xanguinea, and for 

 the rest local forms e. g. of S. spongites show how much this sinus varies. Simil- 

 arly the rosette-plates undergo a considerable variation, as they are sometimes 

 uniporous, sometimes multiporous or the two together and at the same time as 

 the number of pores in the single rosette-plates increases, the number of the 

 rosette-plates themselves decreases. In S. unicornis 2 — 3 rosette-plates with up to 

 ca. 19 pores in each occur on the distal half of each lateral wall. 



In addition to two new forms not described in this work I refer to this genus 

 the following species: >S". unicornis Johnst., S. longirostris Hincks, .S. spongites 

 (Pallas) Smitt, S. sanguined Norman, .S. errata Waters and S. luaperta Michelin. 



Schizoporella longirostris Hincks. 



Schizoporella unicornis, form longirostris Hincks, Annals Nat. Hist. ser. 5, 



Vol. XVII, 1886, p. 266, PI. X, fig. 2. 

 (PI. XVIII, ligs. 3 a— g). 



The zocecia rhombic or trapeziform, strongly arched, with fairly small, densely 

 placed pores, each situated in a deep pit. Proximally to the aperture (here is 

 usually a larger or smaller, often strongly projecting tubercle. The aperture, 

 which apart from the sinus is almost ([uadrangularly oval, is provided with a 

 very faint distal arch, which on each side passes over into a long, high, curved 

 hinge-tooth. The deep and narrow sinus decreases evenly in breadth towards the 

 end. The peristome is represented [)arlly by a low wall, which may surround a 

 larger or smaller part of the aperture and sometimes bears indistinct marks of 

 3 — 4 spines, partly by two small prominent, triangularly rounded projections, 

 which partially cover. the primary sinus and form a secondary sinus; sometimes 

 they almost meet. The strongly chitinized, thick, yellow operculum, which on 

 each side of the accessory portion shows a mark from a hinge-tooth, is proxi- 

 mally provided with a small, rounded, thin expansion. The distal half of the 

 zo(rcium is provided with numerous small rosette-plates, which are placed very 

 close together and each of them is enclosed in a small pore-chamber with thick 



21* 



