1 6 PATTEN AND REDENBAUGH. [Vol. XVI. 



mainly for the attachment of the muscles that move the thoracic 

 appendages and the flexor muscles of the thorax and abdomen. 

 In scorpions and Limulus a complete cartilaginous occipital 

 ring is formed about the spinal cord, near its union with the 

 brain. The plate is a continuous structure, and there is no 

 indication whatever that it consists of two separate lateral 

 bars, as maintained by Gaskell. 



We recognize three parts to the endosternite of arachnids, 

 namely, two lateral bars, a broad plate of cartilage on the 

 dorsal side of the nervous system uniting the posterior ends 

 of the bars, and a bridge of cartilage on the ventral side, which, 

 together with the above-mentioned parts, forms a continuous 

 ring about the anterior end of the spinal cord. 



This whole structure, after the extensive reduction of the tho- 

 racic appendages and the muscles that move them, is retained, 

 apparently, in the ancestral vertebrates, and becomes, as was 

 first pointed out by Patten in '89, the primordial cranium. The 

 lateral bars, the transverse plate, and the ventral arch corre- 

 spond respectively to the trabeculae, the basilar plate, or para- 

 chordals, and the occipital ring. If we add the olfactory and 

 auditory capsules and roof over the remaining dorsal surface, 

 we obtain a complete cartilaginous cranium like that of higher 

 forms. As we shall show elsewhere {American Naturalist), 

 Gaskell's attempt to utilize the endosternite in his version of 

 the origin of vertebrates is a complete failure, since under the 

 conditions of his theory it must be torn apart and put together 

 again upside down, with the old occipital ring replaced by a new 

 one on the opposite side of the endosternite from the one on 

 which it actually occurs. The endosternite has a true carti- 

 laginous consistency, and is composed of a mass of interwoven 

 fibers containing variously formed stellate lacunae, which may 

 be very clearly seen after macerating out the cell contents. 

 The lacunae are united by numerous anastomosing canaliculi, 

 which in the living cartilage contain minute branching proc- 

 esses of the cartilage cells situated in the lacunae. 



II. The six segmentally arranged cartilages of the spinal 

 cord (^\. VI, Fig. i) contain the same kind of cartilage as the 

 endosternite. They are the forerunners, we believe, of the 



