VOL. XLIV.] PHILOSOPHICAL TRANSACTIONS. 327 



viz. 5 double ranks of large and extremely regular cells, as at aa, &c. between 

 which lie 5 other double rows of smaller and less distinct cellulae, shown at bb 

 &c. These cells, which are hexagonal, and of which those in every row lie 

 alternately to those of the next, by which means they fill up the whole space, 

 decrease in their size gradually, as they approach nearer to the top; all the rows 

 at last almost concentring at the apex, leaving only a small space or vacuity, 

 where, in the shells themselves of this kind of echinus, there is an aperture. 

 The smooth part at a is formed of a pebbly stone, bearing the same marks as 

 are usually found in the impression of these echinitae dug up in gravel pits: 

 which proves, that this must have received its general figure from one of those 

 shells, whatever has been the cause of this remarkable configuration of a crys- 

 talline part. 



This curious echinite was found in a marl pit at Baborough, about 3 miles 

 west of the city of Norwich, and was presented to Mr. Baker by Mr. Wm. 

 Arderon, f. r. s. 



On Birds of Passage. By Mr. Mark Cateshj, F. R. S. N° 483, p. 435. 



The places to which birds of passage retreat when they leave us, are first of 

 all to be inquired after, and then it will be proper to examine by what route, and 

 in what manner they convey themselves to such places. 



The reports of their lying torpid in caverns and hollow trees, and of their 

 resting in the same state at the bottom of deep waters, are so ill attested, and 

 absurd in themselves, that the bare mention of them is more than they deserve. 

 Of much the like stamp is a late hypothesis, which sends them above our at- 

 mosphere for a passage to their retreat; which seems as remote from reason, as 

 the ethereal region is from the aerial ; through which last region he cannot con- 

 ceive any obstruction to their passage, when, by the approach of our winter, 

 they find a want of food, and at the same time are directed by instinct to resort 

 to some other parts of the globe, where they may find a fresh supply, for the 

 want of food seems to be the chief, if not the only reason of their migration. 

 And though tit-mice and other small birds abide here the whole winter, and sub- 

 sist on insects, which they find torpid, or in a state of mutation, in the crevices 

 of the barks of trees, and other their winter recesses, yet most birds of passage, 

 having tender bills, are incapacitated for this work ; but then the length of their 

 wings enables them to prey on numberless flying insects, with which the air is 

 stored during the warm months; and it is observable, that not only swallows, 

 but most other summer birds of passage, feed on the wing, on such insects as 

 are seen no more when cold weather commences. 



The various conjectures concerning the places to which birds of passage retire, 

 are occasioned by the want of ocular testimony to bring the matter to some cer- 



