10 



macropora d'Orb. Entalophora ramosissima d'Oib. and Ent. madreporacea Goldf., the 

 Melicerititidae iüways seem to show a disüncl difference from the Cyclostomata therein 

 that the zooecial tul)e the larger part of which is very narrow possesses a great and 

 ratlier sudden distal enlargement while in the Cyclostomata the much wider zooecial 

 tube only gradually widens distally without attaining the great distal enlargement. 

 A transverse section of a cyclostomatous colony therefore shows a number of hol- 

 lows gradually increasing in size from within outwards, (pi. VII, figs. 24, 25), while 

 a corresponding section of a Meliceritiles distinctly shows a contrast between 

 numerous inner very small hollows and a single or double outer circle or series 

 of much larger ones (pi. VII, figs. 1Ü, 19, 23, 30). We see from these transverse 

 sections that the above enlargement takes place especially in the direction from 

 within outwards, and its presence in the Melicerititidae may possibly be explained 

 from the fact that they have possessed an operculum, and a calcareous too, the re- 

 lative great weight of which must have required strong occlusor muscles. We have 

 seen in the historical introduction that Hamm, Hagenow, Marsson and Pergens use 

 the trumpet-shaped enlargement of the zooecia in the Eleidae as a systematic 

 character. d'Orbigny does not mention it, but in his work he has given numerous 

 figures of transverse sections which distinctly show the above contrast between the 

 Cyclostomata and the Melicerititidae. 



The aperture is placed in the distal part of the zooecium which in many spe- 

 cies is more or less protruding. It is always provided with a straight or almost 

 straight proximal margin, and the two lateral margins which are in most cases 

 more or less convex, more seldom somewhat incurved (pi. II, fig. 23.) or almost 

 straight, either run together in a distal curve or form a distal angle (pi. II, figs. 1, 

 23, pi. IV, fig. 22). It takes up a greater or smaller part of the zooecial area, which 

 in a few species (pi. VI, figs. 12, 13, pi. VII, fig. 4) it almost fills, the suboral part of 

 the area being very small. The aperture is in most species surrounded by a more 

 or less developed peristomial thickening a greater or smaller distal part of which 

 is formed by the marginal ridge but in many cases the proximal part of this thicke- 

 ning may be absent or only developed in old zooecia. Sometimes the apertures of 

 contiguous zooecia are divided from each other by broad pillar-like swellings, re- 

 presenting both the dividing ridge, the lateral parts of the peristomial thickening 

 (pi. II, fig. 11). and sometimes also the interoral tubercle (pi. Ill, fig. 18). The præoral 

 tubercle is not rarely developed in the shape of a beak-like projection (pi. I, figs. 

 1,2, pi. V, figs. 3, 4, pi. VI, fig. 13). We have seen in the historical introduction that 

 d'Orbigny characterizes this division by the presence of a calcareous operculum 

 while all the later authors interpret d'ORBiGNv's operculum as a closure-plate. 

 Some of these authors, however, think that these forms have possessed a chitinous 

 operculum, and the presence of an operculum seems, besides, to be a natural con- 

 sequence of the assumption, that certain individuals of the colony must be explai- 

 ned as avicularia an avicularium being a modified zooecium provided with a strongly 

 developed and modified operculum. There can be no doubt that d'Orbigny is right 



