52 I'AL.EONTOLOGY 



internal partitions, causing the shell to split readily across the 

 middle ; and giving rise to deep incisions in those casts of the 

 interior which are so common in the Caradoc sandstone (fig. 



11,8). 



The extinct Spirifcriclce are a family characterized by the 

 possession of internal calcareous spires extending from the 

 centre of the shell outwards (fig. 11, 3). These spires, like the 

 shell itself, are frecpiently silicifiecl, and may be disengaged 

 from the matrix by the action of acid. At other times the 

 shell is imbedded in soft marl, immoveable by careful washing, 

 so as to show the calcareous lamina of the spire fringed with 

 hair-like processes, formerly the support of cirri. In the genus 

 Spirifera the shell has a long straight hinge-line, and the 

 flattened area of the larger valve has a deltoid byssal notch. 

 The typical species are characteristic of the palaeozoic strata, 

 and have a shell-structure like Rlujnchonrfla. The liassic spe- 

 cies (Spiriferina, d'Orb.), have punctate shells, and the byssal 

 opening is closed (at least in the adult) by a thin arched plate 

 or " pseudo-deltidium." In the sub-genus Cyrtia (fig. 11, 4), 

 the hinge-area is ultimately as long as it is wide, and the del- 

 tidium is perforated in the centre by a byssal tube ; some of 

 the species have a punctate shell. The genus Athyris (Dal- 

 man), not always easily distinguished from Tercbratula, has 

 usually a smooth and rounded shell, ornamented with concen- 

 tric lamella? or wing-like expansions (fig. 11, 5) ; the beak is 

 truncated by a round foramen ; the hinge-area is obsolete ; 

 and the spires are as in Spirifcra, with the addition of some 

 further complications near the hinge. There are twenty-five 

 species, mostly from the Devonian and carboniferous rocks. 

 The species of Retzia (King) are still more like plaited Tercbra- 

 tula;, but have lateral spires ; they range from the Silurian 



* The term delthlium, applied by Von Buch to this foramen, has, by miscon- 

 ception of liis meaning, become constantly used for the plates which partially 



close it. 



