2 



BRITISH FOSSILS. 



Echinus than those of the other genera just mentioned. In the 

 disposition of the pores of the ambulacral avenues it approaches 

 Temnopleiirus rather than Echinus or Salmacis. 



No Echinidce with excavated sutures are now known to exist in 

 the European seas or in the temperate and colder portions of the 

 North Atlantic ; they are all inhabitants of tropical seas, and 

 especially of the Indo-Pacific province. The fact of a group of 

 these sea-urchins having inhabited the British area during the epoch 

 of the deposition of the Crags would seem to indicate some ancient 

 relation betAveen that region and seas to the south-east, probably 

 communicating with the Indo-Pacific. 



The body of Temnechinus excavatus is of a depressed melon 

 shape. The ambulacra are a little more than half as broad as 

 the interambulacra. The former are composed of about 82 plates, 

 16 in each series. The three uppermost of these plates are 

 deeply excavated on their inner sides, and bear granules but no 

 tubercles on their more convex portions. The remainder bear 

 each a primary tubercle on the outer margin. They are all ex- 

 cavated more or less on their inner sides, the excavations of the 

 four or five upper ones becoming confluent, those of the remainder 

 forming distinct alternating oblong pits, which become smaller 

 and smaller until tliey are nearly obsolete in the neighbourhood 

 of the mouth. The elevated interstices form a ziczac ridge, the 

 outer angles of which join the bases of the primary tubercles ; 

 on this ridge are minute secondary tubercles and granules. The 

 plates of eacli interambulacral segment are about 20, ten forming 

 each vertical series. The two uppermost joining the genital disk 

 are smooth or nearly so ; the remainder bear each a primary 

 tubercle not larger than those of the ambulacra. The four plates 

 next after the uppermost in each row are tumid on their outer 

 halves, very deeply and steeply excavated on their inner sides. 

 On the inner sides of the tumid portions are the primary tubercles 

 surrounded by secondary tubercles and granules. Their excava- 

 tions combined form a deep ziczac trench, smooth at the base. 

 The remainder of the plates are similarly ornamented with tubercles 

 and granules ; the sutural indentations, however, are not confluent, 

 but form deep loop-shaped smooth pits, becoming very small near 

 the mouth. The lines of division between plates run through the 

 pits. The avenues of pores are deep-set, and are composed of pairs 

 of pores separated by fiiie ridges and ranged in single file, the rows 

 being undulated. There are about 50 pairs of pores in each row. 



