GENUS PYRGOMA. 357 



muscle, the depressions or crests for the other muscles, both 

 on the scuta and terga, are hardly developed. 



Terga : these, as in the case of the scuta, differ so much 

 in shape in the several species, that little can be said of them 

 in common. In P. Anglicum and Stokesii, they are of the 

 normal shape; in P. cancellation (PI. 12, fig. 5 d) this, to a 

 certain extent, is likewise the case, but the spur is produced 

 to a quite extraordinary length. In P. grande (PI. 13, 

 fig. 1 b) there is no distinct spur, and the whole valve is 

 square. In P. milleporce (2/) there is no spur, and the valve 

 is arched and triangular. In P. crenatum the spur is broad, 

 rounded, and depressed (fig. 4 b), with the carino-basal por- 

 tion of the valve reduced to a mere border, barely distin- 

 guishable from the great occludent ledge. In P. monlicularice 

 there is no spur, and the whole valve forms a mere border 

 to the occludent lodge ; and, lastly, in P. dentatum, the valve 

 is extremely variable in shape (fig. 3 c, 3 d, 3/), and on its 

 internal surface (fig. Sg) there is an inwardly projecting, 

 most singular and anomalous, tooth. Hence we see how 

 wonderfully variable the terga are in this genus. 



Structure of the Walls. — The shell consisting, as has 

 been stated, of a single piece, is generally thick. From 

 the close alliance between this genus and Creusia, it is pro- 

 bable that the shell, if examined immediately after the meta- 

 morphosis, would be found to show traces of being formed 

 of four compartments. The walls are either solid or porose ; 

 their basal margin is formed by strong crenated ridges, an- 

 swering to the longitudinal septa in Ealanus; but these in 

 P. monticulari<£ are modified into a very irregular surface. 

 The internal surface of the shell is generally smooth, or only 

 slightly ribbed. The sheath has its lower edge free in P. 

 Anglicum and StoJccsii, and in a slight degree in P. millepora, 

 but in the other species it is closely attached to the walls. 

 In P. monticularice the sheath might easily be overlooked. 

 In P. Anglicum, grande, and crenatum, it descends almost to 

 the basal margin of the depressed shell, and as the opercular 

 valves and membrane are attached to the lower ed«;e of the 

 sheath, the animal's body necessarily comes to be almost 

 exclusively lodged in the cup-formed basis. In P. grande 

 and conjugatum, the lines of growth in the sheath are bent 



