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 (Pl. XI, fig. 10¢, d; Pl. XIX, fig. 3c, d)—a conclusion demonstrated by 
the impressions being divisible into two pairs, as in Waldheimia Austrahs (Pl. XX, 
fig. 117, g); and their agreement, in this respect, with what have never been otherwise 
than looked on as muscular impressions in Zrigonotreta (Pl. IX, fig. 6 a, 6), Leptena 
(Pl. 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 Productus bemg a muscular fulerum; 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 Productus 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 Leptena 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 others this space is considerably narrowed by the approximation of the dental 
plates at their superior margin; while in a fewit has completely disappeared, through 
the dental plates coalescing, and forming an arch-shaped process, as in Camerophoria 
and Pentamerus,' 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 Lepteenas; 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 
1 Fossils showing these modifications are beautifully represented in plates xi and xi, vol. ii, of the 
Geology of Russia. 
