the thorax, seven for the abdomen, and, admitting the same number (as indicated by 

 sense-organs and appendages), seven for the head,=total twenty-one. Now this cha- 

 racter could not be predicated of the Entomostraca ; some had more, some fewer segments. 

 Branchipiis stagnalis, for example, had eleven thoracic and nine abdominal segments, 

 besides the head protected by its cej)halic shield. In Iscmra, in which this shield is also 

 present and of great size, the number of thoracic and abdominal segments exceeded 

 twenty-four. 



Amongst the Trilobites the part of the body next the shield-shaped ceplialic one shows 

 eight segments in Asaphus platycephalus, eleven segments in Phacops, and from thirteen 

 to fifteen in Calymene, besides an abdomen of eight segments. Then there were departures 

 in Entomostraca from the Malacostracous numerical or segmental character by defect as 

 well as by excess, — forms, like Limulus, e. g., with less than twenty-one segments. 



Moreover " the trilobed character of the segments in Serolis and Bopyrus is present 

 also in Limulus, the segments of its body, markedly in the hinder division, presenting 

 three elevations or lobes. The eyes, it is true, are large, sessile, and compound in Cymo- 

 thoa ; but so are the larger pair in Limulus, and more like those of the Trilobite than the 

 eyes of any Isopods are ; the larval Limuli, moreover, roll themselves into a ball " *. 



The value of the nu.merical character of the segments of the body in the question of 

 the affinity of the Trilobites was pointed out by me in a lecture on Crustacea published 

 the week after its delivery, April 27th, 1843 t- 



Burmeister, whose excellent work on Trilobites appeared at Berlin in a later part of 

 1843 X, insisted, with equally original views, on the importance of this character ; but, 

 for his remark that " Limulus was still more widely removed from the Trilobites than 

 the Isopods are " §, I could not see adequate grounds. 



All this, however, is now of mere historical interest ; and I fully concur with my 

 experienced colleague, Henry "Woodward, Esq., F.G.S., whose labours have shed so 

 valuable a light on the afiinities and homologies of the Crustacea other than those in 

 wliich "the normal number of segments is twenty-one," that " the conclusions of Prof. 

 Agassiz and James Hall as to the close affinity existing between the Eurypterida and the 

 Xiphosura are correct "||. Whether the extension of Dana's group, Merostomata, as 



* Owen, ' Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals,' ed. 1855, 

 p. 3.31. 



t In the following terms : — " The distinction between the Entomostraca and Malacostraca in the number of the 

 segments of the body is of the first importance in determining the affinities of the ancient extinct Crustacea called 

 'Trilobites.'" 'Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals,' 8vo, 1843, 

 p. 165. 



j A translation of this work, with notes, by T. Bell & Ed. Forbes, was published by the Eay Society in 1846. 



§ In this view, however, Burmeister received the support of Emmerich in Leonhard und Bronn's Neues Jahrbuch, 

 1845, part i., translated in ' Taylor's Scientific Memoirs,' vol. 4, part siv. p. 253, August 1845. Emmerich defines 

 the " Trilobites as a peculiar order, connecting Malacostraca with Entomostraca, but nearer the latter. They are 

 related to the former by their calcareous crust-like shell, and by their not possessing simple eyes in conjunction with 

 compound eyes. The Woodlice (Isopoda) have, of all Malacostraca, the greatest resemblance to TrUobites." 



II ' Monograph of the British FossU Crustacea belonging to the order Merostomata,' Part I., Palaeontographical 

 Society's vol. for 1866, p. 9. 



