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Show 2 7 2 SIR C. ELIOT ON NUDIBRANCHS [N o v . 2 9 , one specimen being as much as 25 centimetres long and 1G'5 broad. It is very active, swims rapidly, and has been found on the surface of the sea a quarter or half a mile from the shore. The colour of the dorsal surface within the borders is very varying and may be bright light-red, orange, yellow, sandy, almost white, or still more frequently mottled with all these colours. Sometimes the external red border is divided into two parts by a lighter line as in the last variety. The branchiae exhibit somewhat similar variations of colour, but are generally of a pale reddish yellow with darker lines on the axes and white tips to the pinnae. Sometimes the main axes are bright light green. I have been unable to find any differences of structure between these three varieties: fcmstus, mcirginatus, and moebii. In all the radula consists of from 30-45 rows of simply hamate teeth, rather slender, 65-80 on each side of the naked rhachis. The branchiae are from six to eight, seven being perhaps the commonest number. Each so-called branchia consists, as a rule, of four plumes (but sometimes of three or five) inserted very close together but not springing from a common stem. Not unfre-quently one plume is separated a little from the others, and the individual then appears to have an abnormally large number of branchiae. D ortdopsis. This genus is distinguished from the other cryptobranchiate Dorids by having a suctorial buccal apparatus, with no jaws or radula. The mouth is a fine pore situated in the anterior part of the foot; and the internal organs consist of a buccal cone from which issues a long tube which is generally twisted and expands into a dilatation before entering the liver. Beneath the anterior part of the tube is a large folliculate mouth-gland, generally double. The true salivary glands appear to be represented by two nodules at the commencement of the dilatation. The nervous system is very concentrated. The liver is bifid behind. There is an armature of minute hooks on the spermatic duct and glans. On the upper wall of the pericardium are a number of lamellae, sometimes called the pericardial gill. The animals are generally soft, more rarely spiculous, either smooth or tuberculate. The branchiae are rather large and few (rarely more than 8), and though they are completely retractile, are very commonly ex-serted in preserved specimens so as to appear at first sight non-retractile. This feature seems to depend on some peculiarity of texture, and not on any difference of structure. The genus is very abundant in the Indo-Pacific, and about 60 species have been described, many of doubtful validity. Even in dealing with the forms which are adequately described, it is not easy to draw the line between species and varieties, as nearly all are very variable both in shape and colour, and the internal organs present few features which can be safely used for classi |