360 THE CANADIAN NATURALIST. [Yol. vi. 



other independent groups which I have first distinguished under 

 the name of synthetic types. Its resemblance to the Trilobites is 

 unmistakable, and very striking. In the first place the head 

 stands out distinct from the thoracic regions, as the buckler of 

 Trilobites; and the large, kidney-shaped facetted eyes recall those 

 of Calymene; moreover, there is a facial suture across the cheeks, 

 as in Trilobites, so that, were it not for the presence of the anten- 

 nae, which project from the lower side of the anterior margin of 

 the buckler, in two unequal pairs, these resemblances would 

 amount to an absolute identity of structure. As it is, the pres- 

 ence of an hypostome, in the same position as that piece of the 

 mouth is found in Trilobites, renders the similarity to this extinct 

 type of Crustacea still more striking, while the antennae exhibit 

 an unmistakable resemblance to the Isopods. 



In view of the synthetic character of these structural features 

 it should not be overlooked that the buckler of our new crus- 

 tacean, for which I propose the name of Tomocaris Peircei, eX' 

 tends sideways into a tapering point, curved backward over the 

 first thoracic ring, as is the case with a great many Trilobites, 

 The thorax consists of nine rings, seven of which have prominent 

 lateral points, curved backward, like the pleurae of Olenus, Lichas, 

 &c. The sixth rins; is almost concealed between the fifth and 

 seventh, and is destitute of lateral projections, as is also the 

 ninth. These rings are distinctly divided into three nearly 

 equal lobes by a fold or bend on each side of the middle 

 region, so that the thorax has the characteristic appearance of 

 that of the Trilobites, to which the latter owes its name. The 

 legs are very slender, and resemble more those of the Copepods 

 and Ostracoids than those of any other Crustacea. There are 

 nine pairs of them, all alike in structure, six of which, however, 

 the anterior ones, are larger than the three last, which are also 

 more approximated to each other. Besides the legs, there is a 

 pair of maxillipeds attached to that part of the buckler which 

 extends back of the facial suture. These maxillipeds resemble 

 the claw of a Cyclops. All these appendages are inserted in that 

 part of the rings corresponding to the bend of the thoracic lobes; 

 so that, if there exists a real affinity between the Trilobites and 

 our little crustacean, and their resemblance is not simply a case 

 of analogy, we ought hereafter to look to a corresponding position 

 for the insertion of the limbs of Trilobites. I do not remember 

 with sufficient precision what Billings, Dana, and Verrill have 



