Of this embryo the head alone has been represented. Two views of it are given, one (M) from the side and the other (M´) from the under surface. The growth of the front part of the head has considerably diminished the prominence of the cranial flexure. The full complement of visceral clefts is now present--six in all. But the first has already atrophied considerably, and may easily be recognized as the spiracle. In Scyllium, there are present at no period more than six visceral clefts. The first visceral arch on each side has become bent still further round, to form the front border of the mouth. The opening of the mouth has in consequence become still more narrowed in an antero-posterior direction. The width of the mouth in this direction, serves for the present and for some of the subsequent stages as a very convenient indication of age. N. The limbs, or paired fins, have now acquired the general features and form which they possess in the adult. The unpaired fins have now also become divided in a manner not only characteristic of the Elasmobranchii but even of the genus Scyllium. There is a tail fin, an anal fin and two dorsal fins, both the latter being situated behind the posterior paired fins. In the head may be noticed a continuation of the rapid growth of the anterior part. The mouth has become far more narrow and slit-like; and with many other of the organs of the period commences to approach the form of the adult. The present and the three preceding stages shew the gradual changes by which the first visceral arch becomes converted into the rudiments of the upper and of the lower jaw. The fact of the conversion was first made known through the investigations of Messrs Parker and Gegenbaur. O. In this stage the embryo is very rapidly approaching the form of the adult. This is especially noticeable in the fins, which project in a manner quite characteristic of the adult fish. The mouth is slit-like, and the openings of the nasal sacs no longer retain their primitive circular outline. The external gills project from all the gill-slits including the spiracle. P. The head is rapidly elongating by the growth of the snout, and the divisions of the brain can no longer be seen with distinctness from the exterior, and, with the exception of the head and of the external gills, the embryo almost completely resembles the adult. Q. The snout has grown to such an extent, that the head has nearly acquired its adult shape. In the form of its mouth the embryo now quite resembles the adult fish. * * * * *