142 



SYNOPSIS OF THE CHARACTERS OF THE 



from the beak. The appendage at the right side curves boldly forwards, outwards, and towards the right side, 

 Fig. 24. again curving inwards at about the middle of the shell, at which point it gives off a short, blunt 

 process, directed backwards, or towards the cavity of the shell ; this Is, probably, the principal 

 fulcrum for the base of the true arms of the animal ; from this point it again assumes its former 

 direction, until it reaches the front margin of the valve, where it turns inwards and backwards, 

 following the curve of the dorsal valve for about two-thirds the length of the shell, where it 

 again bends forwards, and a little to the right of the first turn, thus forming the first, or basal vo- 

 lution of the spire, which it forms by rejieatlng those curves about eleven times, gradually 

 becoming more attenuated as it approaches the apex, which is obtuse. The appendage of the 

 left side is similar in all things, but takes, of course, an opposite direction. In the annexed cut 

 (fig. 24) I have sketched one of those appendages free from the shell ; a represents the broad, 

 flattened process, by which it is attached to the beak of the ventral or smaller valve, b the 

 short, obtuse process mentioned above ; when in their natural situation in the shell, as indicated by the dotted 

 lines (fio-. 25), the internal surfaces of the broad bases of attachment embrace firmly the external surfaces of the 

 cardinal teeth of the dorsal valve. From the peculiar curves just described at the origin of those 

 appendages, it will be seen a considerable space results between thein, beneath the beaks ; this 

 space aflPords room for the free action of the two upper pair of muscles for closing the valves. 

 For the better understanding the relative position of those parts, I subjoin a profile view of the 

 present species (fig. 25), in which a a represent the origin of the superior adductors ; B B, the ori- 

 gin of the superior adductors in the ventral valve ; c, one of the cardinal teeth of the dorsal valve ; 

 D, spiral appendage. Length one inch, width ten lines. 



Fig. 25. 



Martinia symmetrica, riiil. sp. 



Spirifera symmetrica. Phil. Geol. York. 



Sp. Ch. — Quadrate or subpentagonal, very gibbous, smooth; mesial ridge not defined on the small valve; 

 divided by a sulcus in the middle. 



This species is remarkable for its regular rhomboidal form, and divided mesial furrow, the furrow passes 

 insensibly into the sides of the shell on the small or ventral valve, but is very well defined on the larger one. 

 The surface is quite smooth, the beaks and cardinal area moderate ; the length and width about equal ; depth 

 two-thirds the width. Length one inch nine lines. 



Reticularia. i)/' Coy. 



Gen. Ch. — Hinge-line shorter than the width of the shell; cardinal area triangular; cardinal angles very 

 obtusely rounded ; mesial fold very slightly raised, or none ; surface ornamented with either fine longitudinal 

 or transverse strias, or most usually reticulated by both ; dental lamellae perfectly parallel. 



This beautiful little group includes all those Spirifers analogous to the S. imbricata, S. lineafa, S. micro- 

 gemma, S. reticulata, S. decussata, &c., having a reticulated or striated surface combined with the general 

 form and cardinal area of Martinia, M'Coy, in which genus I formerly placed them, although they obviously 

 formed a very marked group, distinguished by its small size, reticulated or striated surface, and very remarka- 

 bly by the entire absence of the mesial fold in most of the species (in the one or two species which possess a 

 trace of the mesial fold, it is very slightly elevated). But the internal structure which I have recently seen in 

 three of the species, presents a very distinct and important character; the dental lamellae, instead of converging 

 towards the beak, as in all the other forms of Sjiirifer, are in those perfectly parallel to each other and to the 

 central septum, in their whole length, thus confirming by a very interesting internal peculiarity, the easily 

 recognizable external characters. The o-enus is carboniferous and Devonian. 



