2 



BRITISH FOSSILS. 



Of all the extravagant forms of tjiis curious family of trilobites, 

 none seems so extravagant in its ornament as the genns Acidaspis ; 

 the head, thorax, and tail being literally crowded with spines 

 wherever an available angle occurs. The neck segment is produced 

 into 1, 2, 3, or even 8 spines. In the thorax, the segments of the 

 axis have sometimes two long spines on each, and the pleurae have 

 spines on their surface, and frequently two, or even three at the 

 extremity; the tail is found with from six to 25 of these pro- 

 jections, and the margin of the head is generally furnished with 

 a spiny fringe ; to this last character there are but few parallels in 

 the whole family (it occurs in StaurocephaluSy Calymene, and 

 Vyphaspis) ; and it may be compared with the perforated fringe 

 .of Trinucleus, or contrasted with the long frontal spine of Ampyx. 

 Yet, in some respects, Acidaspis resembles Lichas (a genus not yet 

 illustrated in tjiese Decades) in the deep separation of the side 

 lobes from the rest of the glabella, and their frequent fusion with 

 the lateral parts of the head ; here, too, as in Lichas, the facial suture 

 ■cuts the posterior margin of the head. The tail also is composed 

 of but few segments, as indicated by the joints of the axis, for the 

 number of spines on the lateral parts probably do not indicate 

 half the same number of real segments. 



Dr. Emmrich, who wished to show that all trilobites had nearly 

 the same number of body rings — about 20 or 21 — noticed that 

 his genus Odontopleura possessed a much fewer number than tri- 

 lobites in general ; and he proposed to consider the thorax seg- 

 ment as compounded of two, the free joints of which were exhi- 

 bited at the ends of the pleurae. This, however, is not now tenable, 

 for we have seen some species, which on this view would consist 

 of 40 segments, taking the body and tail both into account. It is 

 quite certain that the anterior and posterior divisions of the pleura 

 are both extended, and this character is peculiar to Acidaspis, and 

 to some only of the species. Barrande has shown that the segments 

 of the thorax increase in number with age. The genus is found 

 both in Lower and Upper Silurian, and in Devonian strata. 



Description. — General form broad and depressed, the surface 

 granulose, the edge fringed with radiating spines. The length, ex- 

 clusive of the spines, is eight lines, and the breadth six lines. The 

 head is widely transverse, three times as broad as long, and with 

 the front and back edges parallel ; the cheeks obtuse, squarish at 

 the upper angles, or even overhanging, and fringed with about 

 16 spines, which increase. in size towards the outer margin; below 

 these there is an abrupt contraction, followed by a widely divergent 



