THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE VI. VOL. IV. 



No. IV.— APRIL, 1917. 



ORIGKEILT.A.Xj aeticl. 



I. — Notes on new or imperfectly known' 

 By B. M. Brydone, F.G.S. 



(Continued from the February Number, p} 

 (PLATE IX.) 



A DEVELOPMENTAL sequence of Membrajiiporellce showing 

 progressive subordination of the ribbed front wall to a solid 

 secondary f i-ont wall can be traced fairly directly from Memlraniporella 

 pustulosa, Bryd., 1 which is prevalent in the zone of II. cor-anguinum 

 and the Uintacrinus band and seems to die out in the zone of 

 Marsupites. In the zone (restricted) of A, quadratus it is 

 succeeded by 



Membraniporella thoraciformis, sp. nov. (PL IX, Figs. 1-3.) 



Zoarium unilaminate, incrusting. 



Zocecia of medium size, but very variable, average length from 

 •55 to - 7 mm. ; the primary zocecium is a simple form with a semi- 

 circular aperture surrounded by a thickened rim bearing a blunt 

 median denticle, and an arched front wall arising just within slightly 

 arched side walls and pierced by seven or eight pairs of short narrow 

 slits ; a prominent tubular secondary aperture is formed by general 

 uprising of the margin of the aperture as in M. pustulosa, but the 

 cavities which in that form are left on either side of the marginal 

 denticle are closed in this species ; at the same time a thin horizontal 

 lamina extends from the lower side of the lower lip of the aperture 

 some way downwards over the front wall, and this lamina and the 

 tubular aperture combine to resemble very closely a throat and 

 breast ; at the same time the side walls swell and more or less 

 completely coalesce. 



Ocecia. — The immature and circumferential zocecia have small, very 

 globose ocecia, with an appearance of being perched on the margin of 

 the aperture ; as the secondary aperture develops it embraces the 

 greater part of the ocecia, and its fall development renders them very 

 inconspicuous. 



Avicularia accessory, small, bluntly mandibular, with arrowhead- 

 shaped apertures crossed by a slender bar, placed at the lower side 

 of the secondary aperture and generally leaning up against it in 

 a slightly forward direction ; they are very erratic in occurrence, as 

 the figures show. Although this species appears to be most prevalent 



1 Geol. Mag., 1910, p. 483, PI. XXXVI, Fig. 9. 



DECADE VI. — VOL. TV. — NO. TV. 10 



