350 C. FORSTER COOPER. 



Heteropleuron vialdivense more closely resembles H. bassammi (Gtinther) from Bass' Straits, 

 Australia, than any other species in the genus. It is somewhat similar to this species as 

 figured by Kirkaldy' but differs in the shape of the fins, in being of much smaller size, and 

 in some other small particulars. 



As there is no account of the anatomy of H. bassanum, it is not possible to say whether 

 that species shares the peculiarities of the form here described. 



The largest specimen obtained measured 2 cm.° (H. bassanuin averages 4'3 cm., Kirkaldy). 

 The depth is rather large in proportion to the length (Plate XVIII. fig. 2). The dorsal fin 

 is rather narrow posteriorly but becomes broader as it approaches the head, being at its gi-eatest 

 depth about one-sixth of the total depth of the body. It runs forward with only a very 

 slight notch into the moderately-sized rostral fin. It is throughout its length provided with 

 fin-ray spaces and long fin rays closely crowded together. These number between three and 

 four to the myotome, the rays becoming shorter and slightly broader posteriorly. 



The rostral fin runs forward for a short way and then bends round the snout, and con- 

 tinues ventrally for a short distance until it finally joins the buccal hood on the left side. 

 The ventral fin has the same depth as the dorsal fin in the corresponding region ; it is 

 a continuation of the right metapleural fold, and has single fin-rays and fin-ray spaces, differing 

 in this respect from H. bassanum where they are paired. 



The caudal fin is lance-shaped and, in comparison with Amphioxus lanceolatus, rather 

 narrow. The ventral half is a little deeper than the dorsal. Both the superior and inferior 

 angles are very slight. 



The myotome formulae of the four specimens were 



Number of myotomes from 



The average given by Kirkaldy for H. bassanum is 



75 45 16 14 



Anatomy. This species on the whole is similar to Amphioxus lanceolatus in its 

 larger features, but presents some minor differences. The preoral pit is rather small compared 

 with that of A. lanceolatus, running halfway up the side of the notochord (Fig. 77). Further 

 back it comes to lie high up on the left wall of the buccal hood (Fig. 78). Except at the 

 point where it runs up at the side of the notochord it is very shallow. The blood vessel, 

 which runs above it, is divided at most into two or three vessels, instead of breaking up 

 into a glomerulus as in A. lanceolatus. 



The " Raderorgan " consists of one median dorsal loop, in the left side of which is placed 

 the preoral pit. Each wing of this loop rilns back towards the velum, and then forward 

 again to form the four or more secondary loops on the sides of the buccal hood. The two 

 epithelial tracts join on the base of the velum and complete the ring. 



' Q.J. M.S. vol. XXXVII. (1895). ^ Table II. in Mr Punnett's account of the Meristic Variation. 



