298 Dr. F. A. Bather — Eocene Echinoids from Sokofo. 



further maintained that, as had ah-eady been shown by Mr. V. 

 Gauthier, the ethmolysian apical system was merely a stage of 

 development, connected by imperceptible gradations with the 

 ethmophract stage, and that an individual might be ethmophract 

 in youth and ethmolysian in old age. An objection of this nature 

 could be brought against almost any character, except the few- 

 which may have arisen per sallum. When, however, a slowly 

 evolving character affords the only distinction, and when the 

 intermediate stages are many, as in the present case, it certainly 

 does not seem advisable to base a generic division upon it alone. 

 As a provisional subgeneric denomination for ethmolysian Hemiasters 

 with four gonopores, the name Trachyaster has its conveniences. 

 In such a sense, then, we may say that the specimens from Garadimi 

 belong to Trachyaster. 



Now according to the usual diagnoses, the only distinction between 

 Trachyaster and Linthia (Desor, 1853, genotype L. insignis Merian) 

 is that Linthia has a lateral fascicle passing beneath the periproct. 

 The fact that in the majority of fossils it is very hard to distinguish 

 this lateral fascicle is no argument against its taxonomic importance. 

 But tl)e researches of Mr. Alexander Agassiz seem to show that 

 a lateral fascicle is the remains of what was once a single fasciole 

 enclosing both petals and periproct, the latter structure being nearer 

 the npex in early stages. As the periproct passes downwards 

 from the apex, we may suppose that it carries the posterior region 

 of this primitive fasciole downwards with it ; and then that the 

 peripetalous fasciole is recompleted above the periproct by a secondary 

 posterior half. Whatever may be the function of the fasciole, it is 

 reasonable to suppose that it is interfered with by this transportation ;. 

 hence the appearance of a new fasciole in its place. Therefore one 

 might anticipate for the lateral fasciole an early disappearance ; and 

 that there is actually such a tendency seems to be indicated by the 

 frequent tenuity of the lateral fasciole and by the suppression of 

 portions of it, producing the ' diffuse ' state recogniseti in many 

 well-preserved specimens. The lateral fasciole may therefore be 

 regarded as a degenerate and disappearing structure, and as such 

 it seems an unsafe character on which alone to base a distinction 

 between two forms so extraordinarily alike as Trachyaster and 

 Linthia. This was the opinion of Duncan (1889). 



Fortunately there is another character, far more fundamental and 

 far more constant, and this lies in the heteronomy of interradius- 

 1 on the actinal surface, as described by Loven (Etudes sur les 

 Echinoidees, pp. 50, 51 ; 1875), and as first introduced into the 

 formal diagnoses by Duncan (1889). According to these two 

 authorities, the heteronomy in Hemiaster is of normal type, that 

 in Linthia of ancient type. It is greatly to be regretted that so 

 little attention has been paid to this feature by palgeontologists, 

 and that it is impossible to judge from either descriptions or drawings 

 how far the normal and ancient heteronomies coincide with tlie 

 absence and presence respectively of a lateral fasciole. Specimens 

 of Hemiaster hnfo in the British Museum (registered 34662) from 



