29fi 30 



verse processes originate from (he middle of the vertebra as short, flat triangles, 

 pointing somewhat forwards; the posterior margin is thickened, the anterior quite 

 thin. The inferior arch is situated under the middle, is short and so slender that 

 the large vessels are to a great extent unprotected by skeleton. The verlebræ de- 

 crease in size evenly backwards; the last vertebra (PI. IV, fig. 10) has no inferior 

 arch, and immediately behind the transverse process it divides into two hypural 

 bones, the upper of which is fused to the urostyle. 



Of all the vertebræ, except the 3 anterior, the neural arches are basally pierced 

 by more or fewer rounded openings. 



Ribs are completely wanting. The distal ends of the transverse processes 

 are by connective tissue fastened to the lateral plates of the dermal armour. 



Interspinous bones. As modified interneurals may possibly be regarded 

 the nuchal plates. The anterior smaller one covers the interspace between the 

 occipital crest of the skull and the first spinous process; the posterior, larger and 

 more elongated shield is situated over the spinous processes of the first and second 

 vertebræ (PI. IV, fig. 5). The anterior nuchal plate is provided with a median 

 ventral keel, which — at all events in some individuals — may project rather far 

 down in front of the spinous process of the first vertebra; the posterior also car- 

 ries a median, but longitudinally cleft keel, embracing the lengthened spinous pro- 

 cesses of the two anterior vertebra^. Thus these plates to a certain degree recall the 

 structures found in Aiilostonia where the interspinous origin of the nuchal plates 

 is hardly to be doubted, a transition being found there through an uninterrupted 

 series of structures gradually taking on the shape of ordinary interneurals. In 

 Fistularia this transition is lost, but still some traces of their origin seem preserved, 

 and besides the close relationship between the genera Fistularia and Aulostoma 

 speak in favour of the interpretation accepted here. Now, in Siphonostoma typhle 

 the presence of the ventral keel may indicate the last trace of the interspinous 

 nature, but it may be of quite independent origin, and it is to be remembered that 

 in some Syngnathids, e. g. Hippocampus, the anterior nuchal plate has no keel 

 whatever, and that the relationship with the Aulostomidae is not very close. 

 Furthermore the development of the embryo does not prove anything with regard 

 to the nature of these shields. I have not been able to find them in a cartilagi- 

 nous condition; they appear to originate as bone in connection with the spinous 

 processes, from which they seem to be detached; but, on the other hand, they ap- 

 pear very early, before the median dermal plates are ossified. 



The interspinous bones of the dorsal fin are bisegmented (Pl.IV,fig. 1,8). 

 The proximal (basal) segment is thin, the longitudinal muscular crest is wanting 

 or slightly indicated, the cartilaginous axis richly developed. Close to the upper 

 (cartilaginous) end it sends out a flat wing-like expansion to each side, or two, 

 separated by an incision, sometimes almost closed to a foramen. Upon these ex- 

 pansions rest the medial margins of the upper lateral plates of the dermal armour, 

 firmly attached by connective tissue (PI. IV, fig. 12). Through the narrow apertures 



