11 



ill liis interpretation of the named calcareous plate. The operculum is an arched 

 calcareous plate provided with more or less distinct radiating strife starting from 

 the middle of the proximal margin, and in contrast to the closure-plate found in 

 all the species examined it shows a distinct free margin, while the closure-plate is 

 soldered together with the margin of the aperture, being really a growth starting 

 from this margin and gradually extending over the aperture. Sometimes may be 

 found closure-plates the middle part of which is not yet closed (pi. IV, fig. 6, 15). 

 The very presence of the radiating striae in the operculum is suflicient to show 

 that we have here to do with an independent structure and not with a closure- 

 plate, as such striae which we f. inst. know from the zooecia of the Cheilostomata 

 indicate that the calcification has taken place in radiating belts. At a time when 

 the operculum was still in a membranous state, the calcification therefore started 

 from the middle of the proximal margin and extended in radiating belts gradually 

 outwards and distally. As the other calcareous surfaces the operculum shows a 

 number of more or less distinct pores which in some species are disposed in two 

 distally converging series (pi. I, lig. 1), while in others they are placed in the ends 

 of small claviform projections whicli show a flabelliform arrangement (pi. 1, fig. 11). 

 In Melic. iindala the operculum shows a number of 1 — 4 curved impressions (pi. IV, 

 fig. 11, 12, 16). The operculum fits into the aperture in two different manners. In 

 a number of species the margin of the aperture is in the same manner as a door- 

 frame or a window-frame provided with a more or less developed depression, the 

 "oral ledge" decreasing in breadth towards the proximal margin and destined to 

 support the margin of the operculum, when the zooecium is closed (pi. I, fig. 13; 

 pi. Ill, figs. 10, 11, 23; pi. IV, figs. 18, 22) while in other species the margin of the 

 aperture is only obliquely sloping inwards. The difference between these two cases, 

 however, is not always easy to see, especially when the state of preservation is not, 

 good. — As in most Cyclostomata the zooecial areas as well as the other surfaces 

 are provided with numerous fine pores, but in a few species the pores are only 

 to be found in the opercula, being in the zooecial and heterozooecial areas replaced 

 by more or less numerous pits {Melic. punctata, Melic. pyrenaica). 



, The Heterozooecia. 



d'Orbigny is the first author who has described and pictured the heterozooecia 

 of the Melicerititidae, which he designates as "cellules accessoires" or 'cellules 

 ovariennes" the two terms being generally used by him as synonyms. 



While Marsson and Pergens still look upon these individuals in the same 

 manner as d'Orbigny, namely as gonozooecia Waters is of the o|iinion that they 

 must be regarded as avicularia, and this view has been followed by the later 

 authors. Canu names them "eleocellaires". 



The heterozooecia consist as the zooecia of a long narrow tubiform part hidden 

 within the colony and a superficial part, the heterozooecial area, which is more or 



