DEVELOPMENT OF THE PORBEAGLE SHARK. 165 



intelligible, my drawing of an embryo measuring 454 mm. is 

 reproduced in text-fig. 1. As there is no noteworthy difference 

 in the external features between embryos of .350 to 605 mm., 

 except in point of size, it is possible to make a genei-al summary 

 of the distinctive characters. 



In its natural position the cylindrical body is curled around 

 the massive paunch. The snout is extremely blunt and the 

 whole head dorso-ventrally flattened, both of which characters 

 are in strong conti'ast with the adult condition. There is an 

 internasal groove. The pit beneath the chin shown in text-fig. 1 

 I now believe to be due to the shrinkage of tissues and not a 

 normal character, for I have not observed it in any other speci- 

 men, nor does it appear in Lohberger's excellent photographs. 

 The eyes are well developed. Teeth are prominent in both jaws, 

 even in the smallest specimens examined by me (350 mm.),_^but 



Text-figure 1. 







\ 



Embryo of Lamna. X |. 

 I.G., internasal groove; Y.S., j'olk-stomacli (paunch). 



they are devoid of the lateral cusps which are found on the teeth 

 of the adult. Vestiges of spiracles are usually, but not invariably, 

 present as minute poises situated midway between the eye and 

 the first gill-slit ; in no case yet examined do they communicate 

 with the pharynx. Spiracles in the adult are either absent, or, 

 if present, minute and functionless. The five pairs of large gill- 

 slits are fully open, and the gills well developed (there are no 

 traces of external gills). The lateral line is Avell marked. The 

 lateral keels, so characteristic of the adult, in the caudal region 

 are prominent, as is the notch in the back at the base of the 

 caudal fin. The cloaca is open. The fins are fully developed 

 and resemble closely those of the adult, excepting the caudal, 

 whose dorsal and ventral lobes have not yet expanded so that 

 they present the chelate appearance seen in the flgui-e (at the 



