17 



Genus — Orbitoides, cPOrhigny. 



186. *0. Mantelli, Morton (sp.). Mostly small specimens, com- 

 mon in the Lower Bed. One example is much larger than the 

 average size, and is ^ in. in diameter. A few subglobular ex- 

 amples, surrounded with fringe of delicate translucent chamber- 

 lets, might perhaps be referred to 0. dispansus, Sow. 



187. *0. stellata, sp. nov. PI. i., figs. 9 — 11. Test free, com- 

 pressed, lenticular ; peripheral margin attenuate, angular in out- 

 line, or produced at more or less regular intervals into flat spinous 

 projections. Surface of test rough and vesicular, destitute of 

 areola?, but in well preserved specimens exhibits externally a few 

 tubercles of clear shell substance. Primordial region occupied 

 by one or more chambers of large size ; chamberlets of medial disc, 

 numerous, cylindrical, or roughly hexagonal in section, arranged 

 in concentric annuli ; investing chamberlets compressed laterally, 

 arranged in successive layers on either side of the medial plane ; 

 septal partitions thin, except where columns of clear shell sub- 

 stance pass radially from the primordial region to the external 

 surface of test. Spinous projections formed by intermittent ex- 

 tensions of chamberlets at the peripheral margin. Average 

 size, ^ inch. 



This is an interesting modification of the genus in the direction 

 of Tinoporus, but whilst simulating the latter in external ap- 

 pearances, it is destitute of the supplemental skeleton character- 

 istic of that genus ; and is clearly differentiated from both 

 Tinoporus and Gyptsina by possessing the duj^lex arrangement 

 of segments, which marks-oft' Orhitoides so clearly from these 

 genera. The periphery is sometimes produced into a thin carina 

 composed of a single layer of chamberlets, which from their regu- 

 larity of shape and disposition can be determined as extensions 

 of the medial disc. The spinous projections, whilst seldom entirely 

 absent, vary considerably in their number and arrangement. 

 Specimens occur having the outline of an inequilateral triangle, 

 and pass through every degree of variation in angular outline to 

 that of a symmetrical arrangement of radiating spines, as in fig. 

 9, pi. i. ; the thickness of the walls separating the cliamberlets is 

 relatively much less in comparison with the chamber cavities than 

 is commonly the casein this genus, as shown in pi. i., fig. 106. The 

 primordial region, when examined in flat sections, is seen to con- 

 sist of several large inflated and spirally arranged segments, 

 which are not visible in the transverse section shown in fig. 10«. 

 Figs, llrt — 116 represent a large and very abnormal specimen 

 assumed to belong to this species, having two medial planes 

 growing at right angles to each other, so as to give a triangular 

 or prismatic form to the text when seen in transverse section. 

 Lower Bed, common. 



