106 Professor H. H. Swinnerton — 



crosses the cephalic suture." l Had we spirit specimens to dissect 

 we should no doubt find that the trilohite's eye maintained equally- 

 stable relations to the nerves, vessels, and other organs vitally- 

 connected with it. The position of the eye, then, is a definite 

 morphological point. 



When, therefore, every worker from Burmeister to Raymond in 

 describing the trilobite organization states that the facial suture passes 

 behind the visual area and in front of the palpebral lobe, he is giving to 

 that part of the facial suture a very precise position in the trilobitic 

 anatomy. 



The Ecdysial Line in Meson AciDiE. 



In the Mesonacidse there is no fully developed facial suture on 

 the dorsal surface. Rudiments occur occasionally. Nevertheless, 

 these trilobites must have undergone ecdysis. As long ago as 1891 

 Walcott in describing Callavia Broggeri wrote: "It is a very 

 common occurrence to find the 'doublure' on the reflected under 

 margin lying free from the other parts of the head and with the 

 hypostoma attached. This fact leads to the conclusion that a suture 

 passes around near the frontal margin." 2 He mentions that a 

 similar suture is described by Holm in Holmia Kjerulfi. In 1910 

 the same worker figures an interesting specimen of Pcedeumias 

 transitans (see Figure), which further substantiates these observa- 

 tions. 3 Raymond, referring to this figure, describes the detached 

 portion of the doublure as " swung back so that it presents its 

 ventral face to the observer on the same block with and still 

 attached to the head-shield". 4 This fortunate find makes it 

 possible to determine the morphological relation of this ecdysial 

 line to the dorsal facial suture. Though no true dorsal suture is 

 present the eye-lobes are well shown. If one imagines the doublure 

 replaced in its original position it becomes evident that its line of 

 separation from the remainder of the head-shield must lie either 

 marginally or submarginally, and in any case morphologically, far 

 distant throughout its length from the eye. That is to say, no 

 fraction of it passes between the palpebral and ocular segments, and 

 therefore that no fragment of it is homologous with that portion of 

 a typical facial suture which takes this course. To homologize this 

 line with the facial suture and the detached portion with the free 

 cheeks is merely to cause confusion. For the purposes of this paper 

 these may be called the marginal suture and doublure respectively. 



The Ecdysial Line in Agnostid^. 



The Agnostidse are blind and show no suture dorsally. Beecher 



believed that they possessed facial sutures and free cheeks on the 



ventral side. 5 Raymond claims to have found these in Agnoshis 



?iudus 6 (see Figure). Before applying the results of the above 



1 1894, pi. 420. 



2 C. D. Walcott, Tenth Ann. Rep. U.S. Geol. Surv., 1891, p. 638. 



3 1910, pi. xxxiv, fig. 6. 



4 P. E. Raymond, Amer. Journ. Sci., vol. xliii, p. 208, 1917. 



5 Amer. Journ. Sci., 1897, p. 183. 



6 Ibid., 1917, p. 198. 



