Geographical Distribution of the Terebratulse. 187 



men has been shown to exist, but I find no trace of one in the 

 recent species. The umbo of the ventral valve is largely pro- 

 duced into a thickened spondyloid beak ; and in its front area a 

 large triangular rudimentary deltidium is soldered. The dorsal 

 valve, rudely indented and flattened, is of a horse-shoe shape ; 

 and the ventral valve, densely convex, is rayed outwardly with 

 close-set grooves, which produce serrations on the inner margin. 

 The apophysial system is modified into a fixed ridge in the 

 bed of the valve, accompanied by a profusion of little indenta- 

 tions. When describing Thecidea twenty years ago (Conch. 

 Syst. i. p. 184), I remarked that " the upper valve is flat and 

 curiously indented, as if to fit certain corresponding parts in the 

 body of the animal. These indentations, which spread round in 

 a semicircular direction from the hinge, look exactly as if they 

 were picked out in wax ; and in a specimen which I have ex- 

 amined with considerable minuteness, they were filled with the 

 dried remains of numerous fine cilia." No sort of loop had 

 been observed in Thecidea, and it is now obvious that this in- 

 dented bed of the ciliary arms is a modification of it. 



Geographical Distribution. 



It is difficult to generalize on the geographical distribution of 

 the Terebratulee, with the hope of arriving at many conclusions 

 of interest, without embracing the fossil species. As might be 

 expected in the case of a tribe of animals which existed so much 

 more abundantly in the Silurian seas, and are perhaps destined 

 at no very remote period to disappear altogether, they are much 

 scattered, and are not abundant in individuals. Nevertheless 

 there are few genera of mollusks of which the habitats and specific 

 nature are now so well known. Of the 48 recorded species of 

 Terebratida (cited rather at random by authors at from 60 to 70 in 

 number), 4 are based upon single specimens of somewhat doubt- 

 ful specific value, without any information as to their habitats-— 

 cunceUata, Bouchardii, transversa, suffusa ; but their relationship 

 with undoubted species is not sufficiently obvious to admit of 

 their being discarded. Of 3 species, radiata, supposed to be a 

 native of Corea ; crenulata, of the Canaries ; and dilatata, of Co- 

 quimbo, or the Strait of Magellan, the habitats arc not well 

 authenticated, and appear to me to be doubtful. Of the habitats 

 of the remaining 41 species there is no manner of doubt ; but of 

 two of these the specific value is open to criticism — physema and 

 capsula. There remain, then, 39 species of which the subgenus 

 (founded on the structure of the apophysis) and the habitat may 

 be rehed on. 



13* 



