474 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



creation " of the older writers) that the characters of the species are 

 kept true. In a case like the present, where the centres of distribution 

 of two species of similar habit but slightly diflereot structure are con- 

 tiguous and the two areas quite continuous, there will be an intermediate 

 re"iou occupied by the two species in about equal numbers, whilst a few 

 specimens of one species may extend quite to the centre of distribution 

 of the other and be found living side by side with their now better 

 adapted rivals. They survive because they are in continuous contact 

 with, and are constantly recruited from, their own centre of distribution, 

 where the characters of the species are kept true. Echinus acutus is 

 found living in small numbers side by side with the numerous Echinus 

 esculentus in the Plymouth district, because that district is sufficiently 

 near to a deep-sea area where Echinus acutus is abundant, and to the 

 conditions of which it is specially adapted. Should this communica- 

 tion in any way be cut off, the species would, in all probability, die out 

 in the district. 



Echinus miliaris. Chart V. Present upon practically all grounds. 

 It appeared to be relatively most abundant on Grounds XVI. and 

 XVIL, both of which are clean shell gravel. On sandy grounds the 

 species is generally found clinging to shells, hydroids, etc. 



Habits. A wandering species inhabiting all kinds of grounds. On 

 fine sand generally found attached to hydroids, shells, etc. It has a 

 curious habit of attaching to itself, by means of its tube-feet, bits of 

 shell, small stones, sea-weed, etc., in such a way that it becomes practi- 

 cally hidden by these objects. It lives on food of almost any kind, 

 animal and vegetable, and does great damage on oyster-banks. A small 

 specimen of this species was kept alive in the Laboratory in a shallow 

 dish of sea- water for many months, being fed upon pieces of red sea- 

 weed, which it would eat in large quantities. 



DiSTKiDUTioN. Geograjihical. Northwards to Iceland and southwards to the Mediter- 

 ranean, North Sea [fide Bell, No. 7), Kattegat (Petersen, No. 95), Baltic (Mubius, No. 86). 



Depth. From shore to 50 fathoms (Mcibius and Btltschli, No. 88). 



Bottom-deposit. Forbes (No. 22) records E. miliaris as common on oyster-beds and 

 scallop-banks. Mobius and Biitschli found it on stony ground, on shelly ground, on sand, 

 and on mud ; Meissner and Collin (No. 76) generally on coarse ground, occasionally on 

 sand and sandy mud. 



It is evident that the nature of the bottom-deposit in itself is of no 

 great importance to this species. 



Echinocyamus pusillus. On account of the small size of Echinocyamus, 

 much importance cannot be attached to the results of the dredgings as 

 furnishing a knowledge of its distribution. It seems to be found most 

 frequently upon coarse ground, especially upon clean shell gravel {e.g., 

 Grounds XVI. and XVII.). 



