194 BRITISH CARBONIFEROUS BRACHIOPODA. 



Crania quadrata, M'Cty. PI. XLVIII, figs. 1—13. 



Orbicula quadrata, M'Coy. Synopsis of the Char, of the Carb. Foss. of Ireland, 



pi. xx, fig. 1, 1844. 

 Crania quadrata, Bav. Mon. of Scottish Carb. Brach., pi. v, figs. 12 — 21, 1860. 



Spec. Char. Very variable in shape, on account of its mode of attachment, which is 

 by the entire surface of its lower valve ; when quite regular, is marginally sub-quadrate, 

 almost circular, or slightly elongated, oval, the posterior edge being usually straight, or 

 with a slight inward curve, while the shell is at the same time wider anteriorly than 

 posteriorly. The upper or free valve is conical or limpet-like, the vertex being sub-central 

 and closer to the posterior than to the anterior margin. Externally, the surface is marked 

 with numerous but irregular concentric striae or lines of growth, which give to the shell 

 a somewhat roughened appearance. The interior of the attached or ventral valve is sur- 

 rounded by a raised, thickened border, of moderate width, and upon it the tubular shell- 

 structure is sometimes clearly discernible. In each corner of the disc, close to the posterior 

 inner margin of the raised border, may be seen two somewhat circular, slightly convex, 

 and prominent, but widely separated, muscular scars ; while towards the centre of the disc 

 two other prominent, but appropriate, muscular impressions exist, and which are, at the 

 same time, somewhat hollowed out along their middle. 1 



The interior of the upper or free valve shows in each corner of the disc, close to the 

 posterior inner margin of the border, a convex, oval-shaped, muscular prominence (and 

 which is, according to Mr. Hancock, due to the divaricator muscle), while towards the 

 centre of the valve are two prominent approximate impressions, which are referred by the 

 same authority to the adductor or occlusor muscles. Dimensions very variable ; a large 

 example measured — length 7, width 7 lines. The generality of specimens are, however, 

 much smaller. 



Ob*. The mode of existence peculiar to this as well as to other similarly constructed 

 species is the cause of the great irregularity in shape assumed by the larger number of 

 individuals, for it was the habit of the young of this as well as of other species of the 

 genus to fix themselves as parasites to all kinds of marine objects, and they were some- 

 times so numerously and closely clustered together that their individual regular growth 

 was prevented, from which it can be easily understood that in such cases the animal must 

 have been compelled to develop itself in whatever direction it could find available space. 

 When first formed, and up to a certain age, the shell of the attached valve was exceedingly 

 thin, and adhered so closely to the surface of the object to which it was fixed as to have 



1 Mr. Hancock attributes the two first-mentioned scars to the divaricator, while the central pair are 

 referred to the occlusor ; the other muscular, ovarian, and vascular impressions which should exist in the 

 interior were not sufficiently defined in the present species to admit of their being accurately described. 



