30 THE CRAG POLYZOA. 



1. iNARMATjE. 

 1. M. TUBERCULATA (?) BoSC. PI. II, 'fig. 1. 



Cellulis subovnlibus seu quadrangularibus ; margine sulcato granuloso ; apertura 

 magna orain scabram, subinde denticulatam ostenclente ; supra utrinque tuberculo obtuso 

 munita. 



Cells suboval or quadrangular ; margin sulcate, granular ; aperture large with a 

 rough, sometimes denticulate margin ; a blunt tubercle on each side above the aperture. 



FLUSTRA TUBERCULATA, Bosc, Vers., 2d ed., t. iii, p. 143 (ex. syn.) 



FLUSTRA MEMBRANACEA, Esper, Flustra, pi. v. 



FLUSTRA CRASSIDENTATA, Lamarck, II. n. d. An. s. V., 2d ed., t. ii, p. 224. 



M. MEMBRANACEA, S. Wood, Ann. N. H., xiii ; (var.) Busk, B. M. Cat. 



MEM. TUBERCULATA, Busk, Q. I. Mic. Sc. (Zoophytology), vi, p. 126, pi. xviii, fig. 4. 



Habitat. C. Crag, Sutton ; and Red Crag, S. W., on Mactra ovalis (?) (Recent], 

 Atlantic Ocean, Madeira, Rio cle Janeiro, on Sargassum fluitans. 



The resemblance between the present species and the M. tuberculata, Bosc (sp.), of 

 which abundant examples may be found on the vesicles of Gulf-weed, is so strong as to 

 induce me to regard them as most probably identical, though some not unimportant 

 differences may be pointed out between them. 



The most striking of these is the circumstance that the recent M. tuberculata is found 

 almost exclusively upon Gulf-weed, or, at any rate, upon some species of fucus, whilst the 

 fossil encrusts dead shells. In the form and general appearance of the cells, when the 

 animal matter has been removed by incineration in the recent species, the resemblance 

 between the two is very striking, allowing, of course, for the greater sharpness of the 

 recent form ; but in one particular, a distinction is apparent even in this respect. In the 

 recent M. iuberculata the spines are cylindrical, ascending, and more nearly approximated 

 than they are in the fossil ; in fact, so nearly do they occasionally approach each other 

 in the living form as sometimes to coalesce into a single bifid tubercle. In the fossil, on 

 the contrary, the spines, or rather the remaining bases of them, present an elliptical out- 

 line, the longer axis being oblique with respect to the longitudinal axis of the cell, and 

 they never show any inclination to a nearer approximation. In the fossil form also the 

 spines are often wholly wanting throughout the greater part of a patch, as is shown in 

 c, fig. 1 but in the recent they are invariably present. As the deficiency, however, may 

 perhaps, in great measure be owing to attrition (though of this there is no clear evidence), 

 it cannot be relied upon as a certain diagnostic character. 



