BRACHIOPODA. 



665 



which in the ventral valve is closed more or less completely by a 

 pseudo-deltidium, and in the dorsal valve is occupied by the car- 

 dinal process. The true Spirifers are mainly Silurian, Devonian, 

 and Carboniferous, and the forms of the second of these formations 

 often have the shell winged, or drawn out at the lateral angles (fig. 

 523). In the Permian rocks a few species of the genus are found. 

 The forms included under the name of Spiriferina (fig. 525) range 

 from the Devonian to the Lias, and differ from Spirifera proper in 



Fig. I??.— Spirifera. sculptihs. 

 Devonian. 



-Spirifera mucronata. 

 Devonian. 



the fact that the shell is punctated, and in the presence of a strong 

 median septum in the ventral valve between the dental plates, while 

 the surface of the shell is usually covered with small tubular 

 spines. 



Suessia, of the Lias, resembles Spirifera in general form, and in the 

 impunctate structure of the shell, and agrees with Spiriferina in the 

 possession of a median septum between the dental plates in the ventral 

 valve, while the " crura " of the brachial spirals are united by a transverse 

 band. The Carboniferous Syringothyris, again, resembles Spiriferina 

 in the punctation of the shell ; but the upper ends of the dental lamellae 



Fig. 524. — Spirifera Niagar- 

 ensis. Silurian, America. 



Fig. 525. — Spiriferina ros- 

 trata. Lias. 



Fig. 526. — Spirifera tri- 

 gonalis. Carboniferous 



Limestone. 



are connected with two horizontal plates, which bend downwards in the 

 middle line, and form by their apposition a tubular fissure beneath the 

 beak of the ventral valve. 



In the genus Cyrtia the shell resembles Spirifera in most respects ; 

 but the valves are very unequal, the dorsal valve is approximately flat, 

 and the ventral valve is pyramidal, with a very large triangular hinge- 

 area and a long and narrow foramen, which is partially closed by a 

 pseudo-deltidium. Cyrtina (fig. 527, c and d) resembles Cyrtia in the 

 shape of the valves, but the shell is punctate, whereas in the latter it is 



