Vol. 50.] SYSTEMATIC POSITION OF THE TKILOBITES. 433 



little advance had as yet been made in the study of the arthropoda. 

 While he agreed with Mr. Bernard that the earlier trilobites presented 

 forms with very numerous segments, he pointed out that the later 

 ones showed signs of advance — in having fewer free thoracic rings and 

 a well-developed pygidial shield. He had always cherished the idea 

 that the isopoda might have branched off at some distant time from 

 the trilobita, and he drew attention to such points of structure as 

 the pores in the free cheeks, which were present in such isopoda 

 as Spliceroma and Serolis, and in such trilobites as Phillipsia, 

 Griffithides, Ampyx, and Trinucleus. The way in which the neck- 

 segment is folded around the glabella and forms the free cheeks in 

 both isopods and trilobites must also be deemed significant. 



The discovery of such well-preserved limbs, by Dr. Beecher, in 

 Triarilirus Beckii justified the Author in regarding at least these 

 earlier trilobites as extremely entomostracan in character. 



The Rev. T. R. Stebbing agreed with the Author in thinking 

 that the trilobites have little connexion with the isopods, though 

 the resemblance is sometimes striking, and is often favoured rather 

 than otherwise by the character and position of the eyes. But, 

 whereas the isopods are distinctly malacostracan, with a number of 

 segments never exceeding twenty-one, the number of segments in 

 a trilobite varies as readily as the fashion of a lady's dress. More- 

 over, in many isopods the mandibles are stout and the limbs either 

 strong or long and prominent, making it improbable that the body 

 of the animal should be fossilized without leaving any trace of the 

 appendages, as appears to have happened with the majority of the 

 trilobites. On the other hand, Apus and Lepidurus seem to have 

 still less claim to any close alliance with the trilobites, the two 

 groups being quite devoid of any general resemblance, the phyllo- 

 pods in question having a large carapace extending back over the 

 segments of the thorax, on which the head- shield of the trilobite 

 never encroaches. The tail or pleon of the trilobite is, as a rule, 

 transverse and compact, that of the phyllopod elongate and flexible. 

 Of the phyllopod limbs many are lamellar, while in Walcott's resto- 

 ration of the trilobite Calymene senaria there is a continuous series 

 of legs, all slenderly articulated. If mere guesses are allowable, 

 the suggestion may be hazarded that of living animals the group 

 nearest the trilobites may be the myriapods, as these have a long 

 series of slenderly articulated legs, and segments both numerous and 

 variable in number. The still prevailing obscurity of the subject is 

 illustrated by the fact that Walcott compares certain appearances 

 in his sections of Silurian trilobites with the spiral branchiae of a 

 whale-louse, a parasitic amphipod of probably quite modern deve- 

 lopment. In the figure of a specimen of Triarilirus Beckii, a pair 

 of antennae are represented projecting straight forward from the 

 centre of the head-shield. It may well be wondered where the 

 points of attachment of antennae so placed are to be found on the 

 underside of the trilobite's head. 



Prof. Gr. B. Howes said that he believed the discovery of the 

 terminal anus in the trilobite dealt the death-blow to the association 



