VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 665 



by viewing the southern cross, and observing at what hour it shall appear perpen 

 dicular to the horizon, or rather, when the time will permit, by observing with 

 the plumb line held in the hand the very moment when the stars i^ in the foot, 

 and a. in the head of the southern cross appear equally distant from the perpendi- 

 cular, the latter on the east side, the former on the west: for at the point of 

 time when this happens, there will hardly be an error of 1 minute of the truth, 

 if 15 minutes be added to the hour of mediation of the first point of Aries; 

 which will be determined by the above-mentioned table, the difference of the 

 meridians of the calculator and observer being amended. 



Of two Beautiful Echinites. By Mr. E. da Costa, F.R.S. N° 4Q2, p. 143. 



These 2 echinites are undoubtedly moulded in shells, of a genus of which we 

 at present find some species now living in the seas ; mostly in the West Indies. 

 The echinometra of Aristotle, Aldrovand, and of Dr. Grew, is of this genus. 

 Dr. Breyn calls the whole genus echinanthus; and Mr. Klein, scutum. Wood- 

 ward, in his distribution of fossil echini, calls them the pentaphylloides, from the 

 rays on the upper part forming a beautiful cinquefoil figure ; but erroneously fixes 

 their characteristics in having only one aperture, and that at the basis ; in which 

 he not only contradicts nature, but also the very specimens he quotes in his own 

 collection, which have all two foramens or apertures, and are elegantly figured 

 so by Agostino Scilla, Who was the person that sent them to the doctor; and 

 our late president Sir Hans Sloane has also figured and described 2 species of 

 this genus, one species of which is an inhabitant of our English seas. 



No author has ever described echinites, or stones moulded in the fossil echini of 

 this genus ; nor even have the fossil echini or shells themselves been ever exhi- 

 bited by any lithologist, except by the above-quoted A. Scilla, who found them 

 in Malta, sent them to Dr. Woodward, and to which the doctor in his catalogue 

 recounts 2 other specimens, which were dug up in Maryland ; so rare are the 

 instances of the fossils of this whole genus ! The 2 echinites here described were 

 all found in the midst of some rocks, which were blown up at Port Mahon some 

 years before, and whence they were all brought. 



The first or largest is composed of a hard or stony arenaceous greyish sub- 

 stance, and is of an escutcheon or heart-like shape : it measures about 144- inches 

 in circumference, or quite round the limb or edge, about 2 inches high from the 

 flat or basis, to the tip of the apex, 5 inches in length at the basis, and 4J- in 

 breadth. On the upper part it rises nearly gradually from the edge quite to the 

 apex. A central point, with a slight declining space, tops the said apex ; from 

 which space the body regularly divides into 5 parts figured like leaves to the 

 edge. These leaves are narrow at the apex, greatly widen towards the bottom, 



VOL. IX. '^ Q 



