B. M. Br y done — New Chalk Polyzoa. 147 



extensions over the front wall, into the secondary form ; in this 

 a more or less round, slight^ tabular aperture, partly embracing and 

 partly overshadowing the ooecium, occupies the summit of a con- 

 siderable eminence which has a long gradual slope down to the front 

 wall, a short steep slope, sometimes vertical or even apparently 

 overhanging, down to the succeeding zocecium, and two lateral 

 ridges formed by a pair of avicularia with their beaks pointing 

 inwards ; the primary front wall is left at a considerable depth below 

 the general surface, all except its solid backbone, which is raised to 

 such an extent that it generally touches the secondary front wall at the 

 foot and often unites with it, thus linking it to the secondary front 

 wall at the head by a longitudinal bar ; the apertures left on either 

 side of this bar are quite small, and the ribbed parts of the front wall 

 are generally quite out of sight. 



Ooscia small and globose : as a rule they are wholly enveloped in 

 the eminence formed round the secondary aperture, but Fig. 9 

 illustrates a specimen in which this eminence has not been fully 

 developed throughout and ocecia are partially visible here and there. 



Avicularia very regularly present in pairs below and at the side of 

 the aperture, appearing as swellings bearing on their outer slopes 

 small elongated beaks with arrowhead-shaped apertures : these 

 swellings roof in chambers which go down to the back wall, thus 

 producing the typical Steganoporellan structure ; unpaired avicularia 

 may occur irregularly in other positions. This species occurs in the 

 Norwich and Weybourne Chalk, and persists into the Trimingham 

 Chalk, where it becomes scarce. It has a very considerable general 

 resemblance to some forms of Membraniporella castrum, Bryd., 1 from 

 which it is readily distinguishable by the much greater length of 

 uncovered front wall and the cavity just below the edge of the 

 secondary aperture possessed by the latter. 



Mehbraniporella pyramidalis, sp. nov. (PI. IX, Figs. 10-12.) 



This species is so closely akin to If. transligata that it can be best 

 described by enumerating the points in which they differ. In this 

 species the zocecia are larger, the average length being from '6 to 

 •7 mm. ; the overlap of the primary front wall by the secondary one 

 is more extensive ; the primary front wall has much more slender 

 ribs and looks decadent ; there is only a single cavity of irregular but 

 on the whole semicircular shape in the secondary front wall, as the 

 primary front wall is never attached to it and is almost always 

 wholly out of sight ; the paired avicularia are similar internally, but 

 externally are largely embraced by the apertural prominence, while 

 the beaks stand out more and are longer, the points often impinging 

 on the aperture, and very sharply denned, and give the apertural 

 prominence the appearance of a pyramid, to which they contribute 

 the angles. 



This species is only known to me from the Trimingham Chalk. 



It has attained an almost Steginoporid structure by a route so 



wholly un-Steginoporid that another step along it could hardly fail 



to result in the total suppression of the Cribrilinid front wall and the 



1 Geol. Mag., 1909, p. 398, PI. XXII, Figs. 4, 5. 



