52 



SILURIAN TRILOBITES. 



Young specimens. PL III, fig. 15; PI. IV, fig. 2 (see also Decade 2, Geo!. 

 Surv., pi. i, fig. 7). 



These differ in several particulars from the adult. The head-spines are smaller in 

 proportion, and the head-margin very narrow. The tail, too, is much smaller proportionally, 

 and more pointed, even mucronate, the mucro being in some cases nearly as long as the 

 tail.* The sides of the tail have fewer ribs, and the axis is not so strongly marked out as 

 in the adult. 



In the very neat and characteristic young specimen, PI. Ill, fig. 15, while the general 

 character of var. a is preserved, it is in some respects like the var. y, hereafter described. 



Surface-ornaments and Tubercles. — The tubercles of the surface are larger in propor- 

 tion in the young specimens. On the axis they are particularly strong. In one of Mr. 

 Mushen's specimens (supposed to be a distinct species by De Koninck, and called by him 

 P. Mushenianus, MS.), the second, fifth, and eighth thorax-rings are ornamented by a pair of 

 tubercles, the fourth and seventh rings of the tail-axis, and the first and fourth lateral ribs. 

 Our figured specimen (PI. Ill, fig. 1 5) has the same ornaments, but they are too small to 

 be shown in the figure. This central double row of tubercles is very conspicuous, and 

 must surely have some special meaning. On all the young specimens a distinct central 

 space is marked out by these pairs. On all, the forehead-lobe of the glabella has one 

 pair wide apart and one pair more approximate. Each of the hiuder lobes of the glabella 

 has a pair (in one of Mr. Mushen's fossils even the front head-margin has a pair). 



Then, as before stated, the second, fifth, and eighth thorax-rings show them — the fourth, 



seventh, eighth, and twelfth of the tail, &c. And so we get a median line of double 



Figs. 11, 12. tubercles, which are only more conspicuous in the 



Symmetrical cutaneous glands in the crust of young because in these the tubercles of the general 



ornamented surface are not so large ; and con- 

 sequently these, which do not much increase in 

 size as the animal grows, lose their importance, so 

 far as the exterior is concerned, f 



But that these tubercles are of real significance 

 in the structure of the animal is evident enough, if 

 we examine the fine interior cast figured in PL III, 

 fig. 12. I have reproduced this specimen in the 

 woodcut (fig. 11), and also the exterior of the tuber- 

 cular variety (fig. 12), which is the same specimen 



P. caudatus. 



11. Interior cast from Ledbury. 



12. Exterior of Malvern specimen. 



Pract. Geology. 



Both in Mus. 



This character is in ordinary cases eventually lost, the mucro not growing in proportion to the 

 rest ; but occasionally it is preserved, as in var. h, aculeatus (PI. Ill, fig. 18). 



t M. Barrande has specially noted the same fact, with regard to the Sao hirsuta of the Bohemian 

 basin. 



