362 WILLIAM PATTEN. 



from the vagus mesomeres. (4) They show the same back- 

 ward migration. (5) They are associated with transverse 

 cartilaginous bars or pectoral arches, to which are attached 

 longitudinal muscles arising from the cartilaginous cranium. 

 The transverse bar in Scorpions is quite small, owing to 

 the tactile function of the pectens; but in the gigantic 

 Merostomata the muscles to their oar-like appendages 

 must have been enormous, and the cartilaginous bar for their 

 support not a bit inferior in size to the pectoral arch of Verte- 

 brates, 



The hsemal surface of the cephalothoracic shield of Trilobites 

 is divided into a number of distinct plates, which resemble in 

 shape and arrangement those on the cephalic bucklers of 

 Pterichthys and Bothriolepis (Fig. 13, 1—3). 



The most important resemblances are shown — (1) In the 

 size and general shape of the shields. (2) In the posterior 

 line or row of small plates, which form a cervical suture, 

 and which indicate the presence of vagus segments. (3) In 

 the great semicircular sutures extending parallel with the edge 

 of the shield around the front and sides. (4) In the roughly 

 triangular ocular plates, with the eyes on their median edges. 

 (5) In the facial suture, z. z. (6) In the median row of plates 

 or lobes in which the median eyes are situated. If the reader 

 is not impressed with the resemblances above indicated, I am 

 sure a careful comparison will convince him that the resem- 

 blance is at least much greater than that between the ocular 

 surface of Bothriolepis and Pterichthys and that of a true 

 Vertebrate. 



Now let us compare the neural surface of Pterichthys, or 

 the neural surface of a true fish, with the neural surface of 

 Scorpions and Merostomata, and we shall see that in all these 

 cases the median cranial plates are arranged in pairs, termi- 

 nating in a posterior unpaired plate (Fig. 13, Nos. 4 and 5). 

 If tliese facts mean anything, they show that whenever distinct 

 plates are developed in the cephalothoracic shield of Arachnids 

 and lower Vertebrates the haemal surface will contain an un- 

 paired median row, and the neural surface a paired median 



