2 



BEITISH FOSSILS. 



barely 3, but the cavity on the posterior surface extends for 7 inches. 

 It is, therefore, probable that the membranous fin concealed more 

 of the spine on the back than on the front, a feature found in some 

 of the recent Placoids, with spine-bearing fins. The line of junction 

 between the base and the ornamental portion is less oblique than 

 ordinary, which proves a more erect position of the fin in this than 

 in the other species of the genus. The external surface is closely 

 beset with tubercles, smaller in size and far more numerous than in 

 Asteracanthus ornatissimus. They are arranged in very regular 

 longitudinal series, parallel with the front edge of the bone. They 

 become less numerous as they recede from the front, and cease 

 altogether on the posterior margins. The tubercles are oval, the 

 larger diameter coinciding with the direction of the rows ; on the 

 distal portion of the spine they become smaller and more elongated. 

 They are all ornamented with deep grooves, radiating (in some in- 

 stances spirally) from the apex, the stellate surface being of harder 

 material than the base of the tubercle. The substance of the spine 

 bearing these ornamental projections is very coarse and fibrous. 

 The base is also composed of similar material. It has been already 

 stated that the cavity of the spine extends for nearly two thirds of 

 the back aspect ; the surface beyond this point, which marks the 

 determination of the cutaneous investment, was armed with a double 

 row of falcate processes, in alternating order, a feature common to 

 other species of the genus. Professor Agassiz conjectures that this 

 form of Ichthyodorulite probably belonged to the genus Strophodus, 

 in consequence of the frequent occurrence of teeth of this genus in 

 association with Asteracanthus spines, in the Kimmeridge clay of 

 Shot over, and the oolite of Stonesfield. I am not, however, aware 

 that teeth of Stropliodus have, as yet, been discovered in the Swanage 

 beds. 



Locality. — This form of spine is not uncommon in the Purbeck 

 strata of Swanage and the neighbourhood. The specimen I have 

 selected for the figure and description, belongs to the Dorchester 

 Museum, and was, I believe, obtained with many other fine Pur- 

 beck fossils from Mr. Wilcox, of Swanage. 



Note. — Mr Beckles, of St. Leonards-on- Sea, possesses an Ichthyodorulite, found in the 

 Paludina beds, near Hastings, which varies in some respects from the species descr bed 

 in this article, but it has suffered so much from attrition, thr.t the evidence of specific 

 difference is insufficient. 



Explanation of Plate. 

 Fig. 1. Asteracinthus verrucosus, size of nature. 

 Fig. 2. Tubercle, magnified. 



P. DE. M. Grey Egerton. 



May-XSoX 



