68 



KNOWLEDGE. 



[March 2, 189C. 



means of concealing them. This it effects by covering up 

 the eggs with weed as it leaves them ; thus once more 

 transforming the nest into apparently nothing more tlian 

 a floating mass of weed. In the one photograph wo have 

 the nest as it was left by the bird, while in the other the 

 weed covering the eggs has been removed. This covering 

 of wet, dirty weed answers other purposes, for, in a few 

 days' time, the eggs become so covered with dirt that thoy 

 are exactly the colour of the nest, and thus stand a chance 

 of escaping detection should the bird be forced to leave 

 the nest before it has time to hide its eggs, as sometimes 

 happens. Moreover, the wet weed, by " heating," serves 

 to keep tlie eggs warm, thus allowing the bird to leave 

 tliem for a considerable time if necessary. 



We have dealt with but a few examples of protective 

 resemblance, but we venture to think that enough has been 

 said to show what variety and interest is attached to the 

 subject. 



♦ 



THE LIMBS OF TRILOBITES. 



By Philip Lake, M.A., F.G.S. 



OF all the fossils which are found among the older 

 rocks of the earth's crust, there are none which 

 have attracted more attention than trilobites. 

 Even so long ago as the seventeenth century Lhwyd 

 drew and described certain "figured stones," 

 among which several of these forms may be recognized. 

 The name itself dates from the year 1771, and is derived 

 from the trilobed character of the body. 



In spite, however, of the labours of generations of skilled 

 palffiontologists, little was known of these animals except 

 the shell or "test" of the back. For it is a remarkable 

 fact that, although trilobites are very common in many 

 localities, yet it is only the back that is seen, and in very 

 few cases indeed has the under-surface of the body been 

 observed, or any trace of limbs. There are several ways 

 of accounting for this. In the first place, the limbs may 

 have been soft and dehcate, unfitted for preservation in a 

 fossil condition ; and, in the next place, it is pretty certain 

 that trilobites, like most crustaceans, used to cast their 

 shells periodically, and it is probable that many of the 

 specimens we find are simply these discarded coverings. 



Within the last few years, however, a great deal of light 

 has been thrown upon the structure of trilobites by the 

 discovery of extraordinarily well preserved specimens near 

 Kome, in the State of New York ; and this discovery is so 

 remarkable that it deserves more than a passing notice. 



An ordinary trilobite, such as the " Dudley locust " 

 (Fig. 1), so abundant in the hmestones near Dudley, is 

 made up of three distinct parts. In front is a semicircular 

 shield-like head, usually bearing a pair of eyes ; behind 

 this there are a number of narrow segments articulating 

 with one another, and forming what is called the thorax ; 

 and behind the thorax is another broad shield generally 

 spoken of as the tail. We do not know how far the head, 

 thorax, and tail correspond with the similarly named parts 

 of other animals ; but the terms will serve if we remember, 

 for example, that the thorax does not exactly represent the 

 thorax of an insect. 



Many other crustaceans also possess parts which may be 

 called head, thorax, and tail ; but trilobites exhibit several 

 peculiarities which are not met with in other forms. In 

 every trilobite the body is more or less clearly marked out 

 into three divisions by two longitudinal furrows, which run 

 fi-om the head through the thorax into the tail. Detween 

 the furrows lies the " axis," which generally forms a pro- 

 minent ridge extending nearly the whole length of the 



body ; and on each side of the axis are the more flattened 

 " lateral lobes." 



The head usually presents another peculiarity which is not 

 found in the adult of any living crustacean. In a form such 

 as Vali/iiiiiir it will often happen that even a good specimen 

 does not show the whole of the head, but only the central 

 part. At the same time it will be clear that this is not 







^%m..:.>^-f# 







KiG. 1. — Call/ III rue Bliimenhachi (tlie " Uiullev lofust"). 



due to an accidental fracture, for the separation has taken 

 place along a regular symmetrical line ; and if we examine 

 a complete specimen we shall find this line distinctly 

 marked. The head, in fact, is jointed, and the line of the 

 joint separates the lateral parts of the head from the 

 central portion. This line is called the "facial suture," 

 and in Cdlymene it runs from the front margin to the 

 hinder corners of the head. 



The trilobation of the body and the presence of the 

 facial suture are the two most striking peculiarities of the 

 upper surface of trilobites. The king-crab of the present 

 day in its young state is distinctly trilobed, and also shows a 

 facial suture, and hence many writers were led to conclude 

 that it is the nearest living ally of the trilobites. But this 

 idea is probably incorrect. 



Of the undtr-surface of the body, the only part that is 

 commonly met with is the " labrum " or " hypostome " 

 (see Figs. 4 and .5). This is a broad plate attached to the 

 front margin of the head and reaching backwards as far as 

 the mouth. 



Nearly half a century ago a Russian geologist named 

 Eichwald found certain jointed cylindrical fossil fragments 

 along with numerous trilobites, and he came to the 

 conclusion that these were the antennae and limbs. But 

 they were not attached to the bodies, and most pahi^onto- 

 logists looked upon them with suspicion. 



Other writers found what they supposed to be the points 

 of attachment of limbs ; but it was not till 1870 that any 

 specimen was discovered showing the limbs actually 

 attached to the body itself. It was in a specimen of the 

 genus Asaphus (Fig. 2), in America, that this fortunate 



