1903.] MONSTROSITIES IX FISHES. 5 



Trout embiyos at a corresponding age — three weeks after hatch- 

 ing — is also given. 



The frequency of occuri-ence, the causation, the general appear- 

 ances, and the classification of double monstrosities in fishes have 

 been the subject of many papers and notices, lists of which are 

 given by Prof. B. C. A. Windle in the ' Proceedings ' of the 

 Zoological Society of London, 1895, p. 423, and by Dr. Franz 

 Schmitt in the ' Archiv fiir Entwicklungsmechanik der Organ- 

 ismen,' Bd. xiii. p. 34. The classification adopted here is on the 

 same lines as that of Windle {I. c), and has special reference to 

 the material at my disposal. All mj specimens were examples 

 either (a) of union by the yolk-sac, or (b) of anterior duplicity. 

 The former require only to be mentioned here, as each of the twin 

 bodies contains a complete and separate complement of organs, 

 while the latter may be conveniently divided into — 



Class I. Union in head-region : 



(«) the twin brains united at the optic lobes ; 



(h) the twin brains united at the medulla oblongata. 



Class II. Union in pectoral region : 



(a) the adjacent pectoral fins not i-epresented ; 



(b) the adjacent pectoral fins present, but united and 



reduced in size. 



Class III. Union at posterior part of body : 



(ct) the two alimentary canals united a considerable 



distance in front of the vent ; 

 (b) the two alimentary canals united close to the 



vent. 



Anatomy op Normal Trout Embryos. 



In normal Trout embryos of the same age as the monstrosities, 

 ossification has not yet begun. The parachordal cartilages are 

 uniting round the anterior end of the notochord and have already 

 joined with the trabeculse cranii, which, coalescing in front of the 

 pituitary space, run forwards as a median flattened bar to meet 

 the nasal cartilages. The pittiitary space gives passage to the 

 choroidal and internal carotid arteries and to the back part of the 

 musculus rectus oculi externus (PI. I. fig. 1). On either side, the 

 parachordals have grown upwards in the form of laminar plates, 

 which meet in the mid-dorsal line over the upper part of the 

 medulla, but leave a narrow V-shaped fontanelle over the lower 

 part (PI. I. fig. 2, / 3). The auditory capsules are firm bosses of 

 cartilage, moulded on the labyrinth, closed externally, but widely 

 open towards the brain. Dorsally, they are connected with each 

 other by a thin vault of cartilage roofing the cerebellum ; anteriorly, 

 they are continuous with the supraorbital bars to be afterwards 

 described ; ventrally, they join the trabecular and parachordal 

 cartilages ; and externally they articulate with the hyomandi- 



