CLASSIFICATION OF THE BRACHIOPODA. 



123 



scars formed by the adductor muscles are observable. The anterior pair are approximate, 

 and placed close to the centre, behind which a prominence is sometimes visible in 

 the ventral valve. The posterior pair are situate near the cardinal edge, and are widely 

 separated. The muscular impressions of the attached valve are sometimes shghtly convex, 

 at others deeply excavated ; those of the dorsal valve are convex, the centre pair sometimes 

 developing very prominent apophyses. (See Part II, PI. I, f. 2 b.) The interior of the attached 

 valve is surrounded by a raised and thickened border, exhibiting the tubular shell-structure 

 in a conspicuous manner. The disk of each valve exhibits more or less distinct impressions 

 of the vascular system, which is simply digitated. 



Fig. 44. 



Fig. 45. 



Fig. 46. 



Crania anomala, MiiU,, from specimens dredged oflF Zetland, and communicated by J. S. Bowerbank, Esq. (From drawings 



made by Mr. Woodward.) 



Fig. 44. Upper valve with the animal; the mantle-lobe has been removed, to show the spiral arms and the muscles. The small 

 central depression is caused by the process (c) of the lower valve. The mouth is concealed by the overhanging 

 fringe of cirri. 



Fig. 45. Lower (ventral) valve. 



Fig. 46. Upper (dorsal) valve. Explanation of the letters: a, anterior adductor impression; af , posterior adductors; c,c, points 

 of attachment of the protractor muscles; c', c', impressions of the cardinal muscle; r, r, points of attachment of 

 the retractor muscles. 



The valves of Crania appear to have been opened by the action of sliding muscles, 

 as described by Professor Owen, in Biscina ; those which advance the dorsal valve 

 {protractor muscles), are aided by a single small muscle in the median line, answering 

 exactly to the cardinal muscle of Terebratula. The pair of muscles which bring the free 

 valve back to its place {retractors), are attached outside the anterior adductors of the free 

 valve, and (apparently) to a point between the posterior adductors in the fixed valve (like 

 their equivalents, in Biscma, according to Owen). The two small impressions in the centre 

 of the dorsal valve (fig. 46) are probably caused by the attachment of the muscles of the 

 spiral arms. The labial arms are thick, fleshy, and spirally coiled ; the volutions are 

 few, and directed, vertically, towards the cavity of the dorsal valve, as in Rhi/nchonella and 

 Davidsonia, and unlike Producta and Biscina. The mantle lobes of Crania extend to 

 the edges of the valves, and adhere closely, as in Thecidium ; their margins are plain and 

 thin. 



Obs. Stoboeus appears to be the first author who figured and described a species 

 belonging to this genus, under the names of Numuhs Brattenburgensis and minor} and 

 ^ Stroboeus; Diss. Epist. Lund., 1732, and OpuscvJa, &c. 



