42 F. A. BATHER, GRINOIDBA OF GOTLAND. 



The following description of an ossicle taken from region 5 a^plies in the main 

 to any species, but is based on material afforded by //. Ftetcheri (Text-fin-. 12 and 

 figs. 30—35). 



In section the ossicle is subcrescentic, the concavity of the creseent being towards 

 the inner curvature of the coil. The horns of the creseent are roörtdediy truneate, and 

 on one or both is generally an artlicular depression for :i cirrus, piereed as usual bv an 

 axial canal. The coneave portion of the different ossicles thus forms a shallow groové, 

 lined on either side by cirri. The convexity of the creseent is on the outer enrve of the 

 coil, and is often elevated in a broad somewhat square-shaped median ridge, which, when 

 the stem is tightly coiled, fits into the concavity of the succeeding whorl. 



The upper artieular surfaee of one of these ossicles (Text-tig. 12 (K)) shows a 

 narrow, transversely elongated, and sornetimes slightly curved axial canal; just such a oné 

 as \ve may imagine to have been produeed by the lateral outgrowth of an ordinary round 

 ossicle with round or pentagonal lumen. On the inner side of this axial canal is a well- 

 marked transverse fulcral ridge of the same or slightlv grearter length. fmmediately on 

 the inner side of this ridge, and coextensive with it, is a slight narrow depression. < )n 

 the inner side of this is a smooth transverse band, stretching right across the ossiele; on 

 this no doubt played the slight ridge on the lower artieular surfaee of the succeding 

 ossicle. From either end of the axial canal a faint line is seen to slope towards the outer 

 curve of the ossicle; these lines, together with the axial canal and the outer portion <>f 

 the circumference, enelose a roughly pentagonal or hexagonal area. This area is divided 

 ti]» as follows. From either end of the axial canal a faint ridge runs almost straight ap 

 to the outer circumference. The four-sided spaee thus marked out is again divided by 

 other faint ridges parallel with the two just mentioned. The number and arrangemeni 

 of these latter varv somewhat, but there appear to be two as a rule. The spaces between 

 them are of eoarse appearance, and sornetimes appear to be marked by a few faint trans- 

 verse lines. Thus, two roughly triangulär spaces are contained by the faint lines that 

 slope from the ends of the axial canal, bv the outer ridges that pass up from the same 

 points, and by parts of the circumference: these spaces are clearly marked by aboUt 4 or 

 6 ridges, which are more or less transverse. All the surfaee of the ossicles not occupied 

 bv the above described struetures is finely grann lar. In no case could there be much 

 doubt but that the above described ridges, exeluding the fulcral ridge, indicate the attach- 

 ment of muscles. This indeed was pointed out by Prof. Hall; but it is confirmed by 

 the appearanees of transverse sections. 



A transection through an ossicle at the leve! of the cirrus articulations (Text-tig. 

 12 (5)) shows the following points. The axial canal as before; a canal from either end 

 of it leading to the cirrus; a dense transversely banded mäss of stereom in the region of 

 the fulcral ridge; faint lines, apparentlv sutures, sloping upwards and outwards from the 

 ends of the axial canal; longitudinal and transverse ridges of stereom corresponding with 

 the ridges in the outer quadrangular spaee on the artieular surfaee; stereom absent or 

 verv porous between the ridges. The suture-lines that pass outwards from either end ol 

 the axial canal together with the canal itself separate the ossicle into two unequal por- 

 tions. The härd stereom below the fulcral ridge appears to be the centre ol' calcitication 



