﻿116 CARBONIFEROUS AND PERMIAN FORAMINIFERA. 



Stacheia acervalis, Brady. PI. IX, figs. 6 — 8. 



Webbina acervalis, Brady, 1873. Mem. Geol. Survey, Scotland; Expl. Sheet 23, 



pp. 69, 95, &c. 



Characters. — Test adherent, forming minute, rounded, or elongate patches, com- 

 posed of comparatively few segments. Segments inflated, varying much in size in the 

 same specimen, arranged in an acervuline or indefinite manner. Surface smooth. 

 Length inch (0'8 mm.). 



So far as external form is concerned a more striking analogy could scarcely be found 

 than that of the imperforate Stacheia acervalis and a few-chambered adherent variety of 

 the genus Tinoporus} in the perforate series. It also bears considerable resemblance to 

 some of the less regular Planorbulince. 



Stacheia acervalis was the first variety of the type that could be identified with any 

 certainty as a Foraminifer ; and its smooth imperforate test seemed to suggest an affinity 

 to the adherent Trochammina?, hence its association at that time with the sub-type 

 Webbina. Further study, and the discovery of several allied species, necessitated the 

 establishment of a distinct genus for its reception. 



In external contour this species is very variable, both in the number and form of 

 the segments, and their mode of combination. The cavities of the chambers are often 

 subdivided in the same way as those of S. marginulinoides and S. pupoides ; indeed, the 

 relationship to the latter species is in all respects exceedingly close. The general 

 aperture appears to be situated on the under surface of the test close to the margin, and 

 in one or two examples has been noticed in the form of a slightly produced, bordered 

 neck, as is sometimes seen in the genus Planorbulina. 



Distribution. — In England Stacheia acervalis has only been met with in two 

 localities, both pertaining to the Yoredale Series. In Scotland it occurs in both the 

 Lower and Upper Carboniferous Limestone Groups, though it is much less rare in beds 

 of the former than of the latter age. 



1 Tinoporus lucidus, Brady ('Cat. Brit. Foram. Edin. Mus. Science and Art,' 1870, p. 8, mentioned by 

 name only), a form first noticed in dredgings from the West Coast of Scotland, and since repeatedly found 

 by Mr. Robertson and the Rev. A. M. Norman, as well as by myself. It is a delicate Planorbulina-hke 

 variety, sometimes commencing growth with tolerable regularity, but afterwards becoming acervuline, or 

 else spreading into superficial patches of considerable size. 



