208 



that the membrane covering the aperture has on either side towards the centre 

 a parietal muscle attached to a very fine sclerite. That the areas mentioned, 

 which as it were imitate zooecial surfaces, in reality are bounded by quite a 

 superficial system of furrows and ridges is easily discovered when a layer of 

 zocecia is isolated and the zooecial basal walls are removed by grinding. It will 

 then appear (2 b) that the elongated, hexagonal zooecia are considerably longer 

 and in the greater part of their length only about half as broad as the areas. 

 The oval opening is situated in the distal part of each zooecium and in the proxi- 

 mal part of the frontal surface of each zooecium three or four areas meet. 



The ocBcia, which have been overlooked by Busk, have been found in a 

 number of zooecia in the fragment examined. Each ooecium is situated in the 

 proximal part of a zooecium and may be considered as a part of its cavity, 

 which has obtained a distal and an inner wall of its own. Its frontal wall is a 

 part of that of the zooecium and this is also the case with the proximal wall. 

 The frontal wall is somewhat arched and marked by three, confluent boundary 

 lines between three areas, the marginal ridges of which are a little thickened at 

 this place and have large, closely placed tubercles. Immediately on the distal side 

 of the opercular valve the three-lobed opening of the ooecium appears. My mater- 

 ial has not permitted me to investigate the development of these ooecia. 



Avicularia were not found in the fragment examined; but according to the 

 statement of Busk they occur in small number in the margin of the colony, 

 and as far as we can judge from his figure the greater part of the subopercular 

 area is uncalcified. 



Of this species I have by the kindness of the Director of the British Museum 

 been able to examine a small fragment of the oi-iginal specimen of Busk. 



To this family I must refer the foUowilig species described by d'Orbigny* 

 from the French cretaceous formation, namely Eschara Aceste (PI. 662), E. Achates 

 (PI. 662), E. Acis (PI. 662 & 676), E. Aega (PI. 663), E. Amata (PI. 665), E. Ca- 

 lypso (PI. 669), E. Cgmodoce (PI. 674), E. Dance (PI. 675), Biflustra rhomboidalis 

 (PI. 691), B. mceandrina (PI. 695), B. Elea (PI. 678) & B. echinata (PI. 695). In 

 all these species occurring in free two-laj'ered colonies, quadrangular or hexa- 

 gonal rhombic areas are found arranged in transverse rows, with a larger or 

 smaller, round or oval, generally central aperture, and the avicularia are as in 

 Membranicellaria dubia usually placed in the margins of the colony. Of the just 

 mentioned species I have myself had the opportunity of examining E. Acis & E. 



' 86. 



