332 LECTURE XV. 



find many of the Trilobites to have done before they perished. But 

 we find these characteristics also in Limulus : the segments of the 

 hinder division of its body are divided into three elevations, or lobes ; 

 the large, sessile, compound eyes of Limulus are more like those of 

 the Trilobitethan the eyes of any Isopods are ; and the larval Litnuli 

 roll themselves into a ball. Moreover, the excess in the number of the 

 segments of the thorax or abdomen, over and above the number 

 seven which governs them in the Malacostraca, pi'oves the particulars 

 of resemblance to the sessile-eyed members of that sub- class, which 

 Mr. Macleay indicates, to be no characters of immediate affinity to 

 the Cymothoidce or to the Epicarides, although some of these, like 

 SphcBroma, may present a large, convex, semicircular anal segment, 

 such as we see in the Trilobitic genus Bumastus of the Silurian 

 strata. 



In a lecture on the Crustacea, delivered at the Royal College of 

 Surgeons, April 27, 1843, and published in the following month, I 

 introduced the Trilobites with the following remark: — " The dis- 

 tinction between the Entomostraca and Malacostraca, in the numerical 

 cliaracter of the segments of the body, is of tlie first importance in 

 detei*mining the affinities of those ancient extinct Crustacea, called 

 'Trilobites.'"* And I pointed out how Bopyrus and CymotJioa 

 differed by their normal number of thoracic and abdominal segments, 

 viz., 7 — 7. M. Burmeister, in the same year (1843), with equally 

 original views of the importance of this numerical character, also 

 availed himself of it, in determining the position of these extinct 

 animals in the Systema Naturte, and showed how it invalidated the 

 mai'ks of resemblance which had been noticed between certain Isopods 

 and Trilobites. But he ei'rs, 1 believe, in thinking Limulus to be 

 still more widely removed from the Trilobites than the Isopods are, 

 not apparently knowing the King-crab in its larval state. Professor 

 Emmerich, in a memoir in Bronn's Neuer Jahrbuch, 1845, regards 

 the Trilobites as a peculiar order, connecting the Malacostraca with 

 the Entomostraca, but more nearly related to the latter. He thinks 

 them allied to the Malacostraca by their crust-like shell, and by their 

 not possessing simple eyes together with compound ones ; but this 

 combination is rai'e even in the Entomostraca. He deems the 

 Xiphosura and the Phyllopoda to be the two orders of Entomostraca 

 to which the Trilobites are united by the nearest characters of affinity. 

 They correspond with both as regards the form and size of the 

 clypeoid head or cephalo-thorax. The soft texture of the under side 

 of tlie body of the Trilobites, and the circumstance that no one has 



* LXXXIV. p. 165. 



