76 PERMIAN FOSSILS. 



Two of the four impressions under consideration (probably the anterior pair) I 

 consider to be due to the valvulars, and the others to the cardinals. The former 

 muscles, according to this view, have necessarily produced the dendritic impressions 

 generally to be seen on the flat or opposite valve adjoining, and on each side of, the 

 median plate (PI. XI, fig. 10 c, d\ PI. XIX, fig. 3 c, d) — a conclusion demonstrated by 

 the impressions being divisible into two pairs, as in Waldlieimia Australis (PI. XX, 

 fig. 1 \f, g) ; and their agreement, in this respect, with what have never been otherwise 

 than looked on as muscular impressions in Trigonotreta (PI. IX, fig. 6 a, b), Leptcena 

 (PI. XX, fig. 7 c, d), and several well-known species of Orthis and Strophomena. 



Sufficient has already been advanced in proof of the boss on the hinge of the small 

 valve of Frodudus being a muscular fulcrum ; but to which pair of the four central 

 muscular impressions on the opposite valve it was related, is far from clear ; should 

 it prove correct, however, that the anterior pair was produced by the valvulars, it will 

 follow that the posterior pair has been caused by the cardinals. 



Thinking that a restoration of the myology of Frodudus would be acceptable, I 

 have been induced to give representations of it under figures 4 and 5, in Plate XIX, 

 which will also tend to render intelligible the foregoing observations. A reference to 

 the explanation of the figures on the page opposite the plate containing them will con- 

 veniently make known the various muscles as restored in accordance with the views 

 brought forward.' 



There is one peculiarity which must not be overlooked in a consideration of the 

 muscular impressions of the Brachiopods ; namely, their liability to become changed 

 into projecting apophyses. In young specimens of Leptana analoga the muscles of the 

 large valve have evidently been implanted in a shallow excavation in the substance of 

 the shell ; but in full-grown specimens the excavation has become modified into a 

 saucer-shaped process, strikingly resembling the convex valve of a Strophomena, being 

 marked on the side facing the inner surface of the valve, to which it is attached, with 

 fine, radiating, somewhat distant lines. A similar modification is to be seen in various 

 species of Strophomena. In the rostral valve of certain Schizophorias and Orthisinas, 

 the muscular impressions are situated on a flattened space between the dental plates ; 

 in otliers this space is considerably narrowed by the approximation of the - dental 

 plates at their superior margin ; while in a few it has completely disappeared, through 

 the dental plates coalescing, and forming an arch-shaped process, as in Camerophoria 

 and Fentamerus} which process, in all shells provided with it, must be regarded as a 

 true muscular fulcrum. In the opposite valve there is a similar tendency to modification 

 displayed, though not so often. It is evidenced in Leptsenas ; singularly so in the 

 base of the arched plate already noticed in Strophomena Dutertrii ; and, as will be here- 

 after shown, it has induced the formation of the spatula-shaped process, already noticed 



' Fossils showing these modifications are beautifully represented in plates xi and xii, vol. ii, of the 

 Geology of Russia. 



