belonging to the Class Palliobranchiata. 39 



situated as in Leptana, in the beak itself, but in the apex of the 

 cicatrix of the deltidium : in this respect Strophomena alternata 

 corresponds with Orthis anomala and Spirifer heteroclitus. The 

 foramen in Leptana analoga becomes closed in old individuals, as 

 occasionally occurs in Terebratula. 



It is highly in favour of there being a valid generic distinction 

 between Strophomena and Leptana, when, out of a large number 

 of species at present known, there is little difficulty in placing 

 them either in the one genus or the other. Though I am not 

 aware of it being the case, it is nevertheless probable that species 

 will yet be found rendering a generic allocation a matter of some 

 difficulty. 



Previously to entering upon the next genera to be noticed, it 

 will be necessary to make a few remarks on the muscular system 

 of Terebratula, in order that the use of certain parts to be men- 

 tioned hereafter may be properly understood. From a specimen 

 of Terebratula dorsata, at present before me, containing the entire 

 muscular system desiccated, and freed of the visceral mass, I have 

 drawn up the following details : — The rostral or umbonal cavity 

 is occupied with a dense fibrous cylindrical body called the pedicle : 

 considering the convexity of the foraminal valve as the upper side 

 of the shell, the inferior end of the pedicle fits into the foramen ; 

 while its superior end, which is somewhat flattened or dilated in 

 the transverse direction of the shell, is situated at the entrance 

 or anterior part of the rostral cavity, to the surface of which it 

 appears to be attached by means of tendinous or membranous 

 chords, — the truncated extremity of the pedicle itself not being 

 adherent. A little in advance of the upper extremity of the pe- 

 dicle, three pairs of muscles pass off to different parts. The out- 

 ermost pair (which consists of those muscles implanted nearest 

 the lateral margins of the valve) passes at a slight angle into the 

 upper part of the pedicle : within these muscles, and somewhat 

 in front of them, another pair passes downwards (slightly con- 

 verging at the same time), and becomes attached to a flattened 

 prominency situated in the centre of the hinge of the lower or 

 imperforate valve. To distinguish these pairs of muscles from 

 each other, it will be necessary to name the former the superior 

 pedicle muscles, and the latter the cardinal muscles. In close 

 proximity to the superior end of the pedicle, and a little be- 

 hind, and within the cardinal muscles, and therefore near the 

 mcdio-longitudinal line of the shell, is situated the origin of 

 the remaining pair, which passes directly down to a little behind 

 the centre of the opposite valve, each muscle at the same time 

 becoming dichotomous in its inferior half : these may be termed 

 the valvular muscles. Besides supporting the cardinals and the 



