498 PROF. OWEN ON THE ANATOMY 
In these the cephaletral shield was small, both in breadth and length, as compared with 
that in Limulus and Bellimurus; but it was similarly shaped as regards the curved 
anterior trenchant fossorial margin. The mouth was inferior, bounded laterally by the 
carding-joints, and posteriorly by a *labium,' or connate chilaria, of large size—and if 
homologous with the parts in like relation to the mouth of Limulus (*, *, Plates XXX VIT, 
XXXVIII.) differing therefrom by the pair exhibiting the condition shown by the 
thoracetral limbs of Jimulus. Whatever homology be adopted, the hindmost of the 
, trophi,” or oral organs, is single and symmetrical in Eurypterids. No fossil Merostome 
has yet been discovered showing more than three pairs of cephalateral jaw-limbs between 
the foremost and hindmost pairs. Thus there is one pair less than in Xiphosures. In 
the fine fossil exuvium of the young Pterygotus anglicus figured by H. Woodward in 
his excellent *Monograph on the Merostomata” (plate ii. fig. 1) *, there seem to be as 
many as five limbs on the left side, with spinigerous haunches; but it is uncertain 
whether the foremost of these may not be the fellow of the second, displaced from the 
right side. This, therefore, leaves the forcipate antennæ, or foremost pair of jointed 
cephaletral limbs, devoid, like that pair in Limulus, of the basal carding jaw-plate. 
I think it of less moment to speculate as to which of the six pairs (11-vi1) of cephal- 
etral limbs in Xiphosures were undeveloped in Eurypterids, than to realize the certain 
correspondence of character of the five developed pairs in the latter family with those 
attached to the Limuline cephaletron. As in Limulus, moreover, a partially coalesced 
pair of opercular plates extended backward in Pterygotus from the under and hinder 
border, more or less concealing the underparts of the two anterior segments of the 
thoracetron. 
The foremost division of the body, in both Zurypterus and Pterygotus, is doubtless 
composed of fewer segments than in Limulus. The next division of the body, in Eury- 
pterids, includes a greater number of segments ; and the broadest of these but little exceed 
in that dimension the coalesced cephaletral segments. The whole body of the extinct 
Merostomes is longer and narrower, exhibiting less of concentration and more of irrela- 
tive repetition, than iu the existing Limuline form. 
If shape and relative size affect so little the conclusion above supported of the ABUS 
logy of the cephaletron in Xiphosures and Eurypterids, much weight cannot be attached 
to the difference of form and proportions of the * pleon' or ‘telson’ in the same question, 
especially with present knowledge of the intermediate modifications of this division of the 
body, as seen in Eurypterus and Stylonurus. As the cephaletron of Limulus includes 
more segments and appendages than does that of Pterygotus, so likewise may the pleon 
of Pterygotus as compared with that of Limulus. The excess of segments of the thorac- 
etron in Eurypterids (which excess H. Woodward is disposed to refer to another division 
of the body, which he terms * abdomen") may be among those of which embryologists 
of Limulus believe themselves to have seen traces in its budding tail-spine. However 
* Vol. of the Paleeuntographical Society for 1866. 
+ Nieszkowski, Der Eurypterus remipes aus den obersilurischen Schichten der Insel (Esel" Archiv. für die 
Naturkunde Liv- Ehst- u. Kurlands, Erste Serie, Bd. ii. tab. i. fig. 1"; quoted by Anton Dohrm, loc. cit. p. 640, Taf. 
xiv.fig. 21. H. Woodward, Eurypterus brodiei, from Perton, * Quarterly Journal of the Geol. Society, March 1871, 
p. 261, figure 1. 
