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Show 8 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, Fig. 2 represents the ventral aspect of the pair of fins afore named, as they lay in life. They were attached to the pelvic cartilage (pl.) by a fibrous buffer, identical with that described by Davidoff" (7, p. 124). The free end of the hip-girdle terminated in front in a pointed extremity (processus impar of Davidoff), which, as already observed by Gunther (14, p. 535) and that author (7, p. 124), was bent towards the left side. I figure this (fig. 2 a), as its distortion is here much more marked than in any specimen yet drawn. According to Schneider (23, p. 521) "Die Curve, welche der dorsale Rand jeder Flosse beschreibt, ist verschieden von der Curve des ventralen Randes. Nun ist der dorsale Rand der einen Flosse congruent mit dem ventralen Rande der anderen." In the specimen here figured, the two fins were sickle-shaped; the inner half of the preaxial border of the left one was straight, as represented in the figure. It will be observed that as they lay flattened out, their free ends were both directed towards the animal's right side ; the excavated border, which imparts to the fin-lobe its sickle-shape, was preaxial for the right fin, postaxial for the left. When applied to the sides of the body, the apex of the former looked dorsally, that of the latter ventrally. The contour of the Ceratodus fin is variable ; occasionally its opposite margins are symmetrical with respect to the axis ; but the differences in symmetry between these two fins more than cover those which I have observed between any two members at m y disposal. Turning now to the supporting skeleton, it will be seen that the second mesomere bears, as Schneider has pointed out, an accessory lobe (his "anderes Stuck " referred to above). That, however, instead of being symmetrical, as he claims it to be, is, in this specimen, unsymmetrical to the utmost-for the right fin it is postaxial, for the left one preaxial. Further comment is needless, as the drawing which I give speaks for itself. Thus far the characters of the pelvic fin, as defined by Schneider, are seen to be inconstant and untenable : more than that, however ; for, in that the preaxial lobe of the one fin corresponds almost to a degree (with the exception of one feature, to which I shall return) with the postaxial lobe of its fellow and vice versci, there are embodied in the two the more important differences held by him to exist between the pectoral and pelvic members. Schneider goes on to say (p. 523), " wenn man die symmetrische Stellung der vorderen und hinteren Flosse in Betracht zieht, so leuchtet die Aehnlichkeit des ersten Gliedes des Hauptstrahls mit Humerus und Femur des zweiten Gliedes des Hauptstrahls mit Ulna-Radius und Tibia-Fibula ein." I have shown above that the characters of this "zweites Glied " are inconstant for the pelvic fin. Its accessory lobe is present on that side on which the parameres are stoutest, be it preaxial or postaxial ; and examination of the specimen under m y hand suggests unmistakably that it has arisen as the result of coalescence between the second mesomere and the confluent bases of the two proximal parameres. The well-known lobe of the pectoral member (cf. figs. 5 and 6, mt.), first accurately |