208 



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

 a parietal muscle attached to a very fine sclerite. Thai llie areas mentioned, 

 which as it were imitate zod-cial surfaces, in reality are hounded by quite a 

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

 zooecia is isolated and the zotvcial basal walls are removed by grinding. Il will 

 then a])pear (2 1)) Ihat the elongated, hexagonal zoa-cia are considerably longer 

 and in the greater i)art of their length only about half as broad as the areas. 

 The oval opening is situated in the distal part of each zo(i>cium and in the proxi- 

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



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

 number of zoo'cia in the fragment examined. Each ooecium is situated in the 

 proximal part of a zoa*ciuin 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 (he 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 oa?cium 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 Husk they occur in small numlier 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 original specimen of Busk. 



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

 from the French cretaceous formation, namely £^sc/iara Aces/e (PI. 662), E. Achates 

 (PI. 662), E. Acts (PI. 662 & 676), E. Aega (PI. 66;^, E. Amatct (PI. 665), E. Ca- 

 hipso (PI. 669), E. Cijmodoce (PI. 674), E. Dana- (PI. 675), Hiflustra rhnmbnidalis 

 (PI. ()91), B. mccandrina (PI. (H)5), B. Elea (PI. 678) & B. echinata (PI. 69.")). In 

 all these species occurring in free two-layered 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 

 Menibranicellaria dubid 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. 



