Dr. R. H. Traquair — On Diplacanthus, sp. nov. 255 



The pectoral spine is well shown in the Hugh Miller specimen, 

 and has the same sickle-shaped contour as that in D. striatus ; but 

 the ridges are, as in the case of the dorsal spines, less numerous 

 and more flattened. The secondary stiiation is not observable on 

 this spine, but it is most distinctly seen on the anterior dorsal one, 

 which lies close to it in the same nodule. The posterior or concave 

 border of the pectoral spine is furnished with recurved denticles, 

 a character also found in D. striatus, though not noticed by either 

 Agassiz or Smith Woodward. 



The spines seem rather smaller in proportion to the dimensions 

 of the fish than in D. striatus. The scales are quite similar in. 

 character. 



I propose to name this species Diplacanthus tenuistriatus, and its 

 recognition is certainly not complicated by any claim to revival 

 on the part of any of the cast-aside synonyms of D. striatus. 



Formation and Locality. — Old Eed Sandstone of the Orcadian 

 series ; Cromarty and Gamrie. 



Eemarks on the Acanthodian Shouldek-Giedle. 



It has long been well known that the pectoral spine in Acantliodes 

 was supported by a peculiar bone constricted in the middle, ex- 

 panded at both extremities, and perforated along its long axis 

 by an internal hollow or canal. This ossicle has usuall}^ been 

 interpreted as an element of the shoulder-girdle, and designated 

 " clavicle," or " coracoid." 



Mr. Smith Woodward, however, in the second part of his 

 "Catalogue of the Fossil Fishes in the British Museum," published 

 in 1891, put forward an entirely different theory as to the nature 

 of this bone. Instead of considering the bone in question to belong 

 to the shoulder, he looks upon it as a " basal " element of the fin. 

 In fact, as I understand the case, Mr. Woodward regards the pectoi'al 

 fin of Acantliodes as specialized from the Selachian type by the 

 abbreviation of the Basipterygium, and finally by the disappearance 

 of all the basals save one. 



. The configuration of this element in Acantliodes certainly suggests 

 strongly the idea of a bone, preformed in cartilage and ossified from 

 the outside inwai'ds, and leaving a conically constricted internal 

 cavity. But there are other Acanthodian genera than Acantliodes, 

 and in some of these, notably among the Diplacanthidee, the form 

 of the bone in question is considerably different. In Diplacanthus 

 the bone does not present that hour-glass sort of shape we are 

 familiar with in Acanthodes, but is pointed above, expanded and 

 flattened below, so as to assume an appearance reminding us of a 

 Pal^oniscid clavicle. And in fact Mr. Woodward does interpret 

 this element in Diplacanthus as a " clavicle," and has pointed out 

 two small " infra-clavicular " pieces which connect in the middle 

 line the lower extremities of the clavicles of the two sides. More- 

 over, he makes the possession, or not, of a clavicle into a taxonomic 

 character : thus the Acanthodidas have one dorsal fin and no clavicles ; 

 the Ischnacanthidae two dorsal fins and no clavicles; the Diplacan- 



