BRACHIOPODA 55 



Cyrtia, but lias no hinge, and neither foramen nor internal 

 processes, except a row of small projections along the hinge- 

 line, and two small lateral groups of ridges in the smaller 

 valve. The interior is punctato-striate, but has no recognisable 

 muscular markings. 



The Produdidce are altogether palaeozoic fossils, and most 

 abundant in the carboniferous limestones. Their valves are 

 concavo-convex, the hinge-line straight, and the interior 

 marked with distinct impressions of the muscles for opening 

 and closing the valves, and simple vascular spaces. There are 

 oO species of Produda found in the upper palaeozoic rocks, and 

 having a very wide range in North and South America, and 

 from Spitzbergen to Thibet and Tasmania. Some of them 

 are extremely variable in form ; many are armed with long- 

 tubular spines, and others completely clothed with short, 

 hair-like processes ; they have no hinge-teeth, and the hinge- 

 area is extremely narrow, except in the sub-genus Aidosteges of 

 the Eussian zechstein. Produda proboscidea has its convex- 

 valve prolonged into a tube, as if for the constant supply of 

 respiratory currents. The Permian genus Strophalosia has its 

 valves articulated by hinge-teeth, and covered with long and 

 slender hollow spines ; the shell is attached when young by 

 the umbo of the large valve. Chonetes is distinguished from 

 Produda by a row of spines along the hinge-margin of the 

 convex-valve ; it also has a narrow hinge-area with a covered 

 notch, and small hinge-teeth. There are 25 species in the 

 Silurian and carboniferous, usually of small size, and finely 

 striated. 



Crania is one of the oldest living types, ranging upwards 

 from the lower Silurian. One of the earliest species appears 

 to have been unattached, and another to have had hinge-teeth. 

 Crania Ignabergensis, of the chalk of Sweden, has the valves 

 externally alike, being attached only when very young. The 

 internal markings of C. antique, and other fossil species, are 



