68 TRANSACTIONS OF THE [NOV. 16^ 



the peculiar terminal of the new species of Edestus, one cannot 

 at present help feeling that its position may have been some- 

 where on the head region, perhaps above or behind the eye in 

 some early type of Cestraciont. For it is certainly more natural 

 to place Edestus within a group of known spines than to at- 

 tempt to create for it a station by itself. In the former case 

 the evolution of the remarkable head spines would receive a 

 plausible, if not probable, solution. Beginning with the simplest 

 type of Edestus, situated, doubtless, far back, within the met- 

 ameral conditions of the anterior muscles of the trunk, the spine 

 has increased in curvature, at the same time tending to occur 

 further and further forward.* A degree of asymmetry may 

 next have appeared, and the peculiar forms of Listracanthiis 

 and Erismacanthus may have culminated the series. At all 

 events this is a suggestion which appears to be otfered by the 

 present comparison of the forms of Edestus. 



The further conclusion afforded by the present comparison 

 bears upon the problem of the origin of spines in primitive 

 gnathostomes. Or, to be more accurate, upon one mode of 

 spine origin, for everyone would be inclined to admit that there 

 may have been several. In the present case the evidence may 

 be accepted as conclusive that a spine-like organ had its origin 

 as a metameral structure whose basal portion lay within the in- 

 tegument, and traversed longitudinally a number (seven at least) 

 of body segments. And that from this condition arose a more 

 or less typical spine shaft, thick at one end and pointed at the 

 other, with indications that its decurved character was accom- 

 panied by a firmer insertion of the proximal end, and the 

 eversion of the pointed tip. In this event it is but reason- 

 able to assume that the principal difficulty in accounting for 

 the origin of a spine has been overcome, for we have in Edestus 

 the actual stages in which a horizontal structure belonging pe- 

 culiarly to the body wall has become so decurved that its proxi- 

 mal end is certainly well nigh vertical. And when once a struc- 

 ture has been evolved which is evidently spine-like, one must 

 admit that the straightening and lengthening of its shaft — pro- 

 cesses accompanied by the fusion of the segmental elements and 

 the shortening of the basal line of insertion within the integu- 

 ment — and its perfected dermal encasement and ornamentation 

 become relatively at all events matters of secondary importance. 

 In other words, the present writer is led to believe on the evi- 

 dence of the spine series in Edestus that the origin of many of 

 the dorsal and head spines of the early gnathostomes, the Ces- 



*This tendency Is weU known among fishes, e. g., Pleur acanthus, Chimaeroids 

 and many teleosts, Lophius, Trachypteriis, Bregmaceros, I'atcecus, flat fishes. 



