50 C. R. Eastman — Preliminary Note^ etc. 



cular attachment. In contrast to this, it should be noted that 

 the surface of the spoon-shaped concavity is perfectly smooth. 



The question naturally arises, what purpose could this termi- 

 nal structure have subserved % Was it in relation with the 

 muscular system of the body, or for the support of a movable 

 dorsal spine, such as is found in the recent Siluroids, for in- 

 stance ? It is easy to believe that the hollow part of the pos- 

 terior process just described served equally well either for the 

 lodgement of a ball of muscle or cartilage, or for the reception 

 of the articular base of a spine, while the deep furrows above 

 and below were for the insertion of tendons. The nearly per- 

 pendicular slant of the axis of the concavity, its shape and 

 size, and the smoothness of its walls, strongly suggest that a 

 dermal spine was implanted in it ; but as yet we have only 

 negative evidence of the presence of these defences in Dinich- 

 thys. We might theoretically associate with this genus cer- 

 tain detached ichthyodorulites coinciding with it in dis- 

 tribution throughout the same horizon ; but more than a 

 mere correspondence of parts is necessary before giving cre- 

 dence to such a speculation. Hence we can only affirm that if 

 the posterior process of the dorsal shield supported a spine or 

 spines, we are ignorant of their nature. 



On the other hand, we know that the paired fins of Dinich- 

 thys were more or less rudimentary ; and the only means for 

 changing the direction of motion in the vertical plane while 

 swimming was by flexure of the hinder part of the trunk and 

 tail. It is evident that muscles inserted along the posterior 

 process of the dorsal shield, and continued down the back on 

 either side of the neural arches, would on contracting produce 

 an upward flexure of the caudal region, owing to the arched 

 curvature of the body. The effect of such a flexure is equiv- 

 alent to elevating the head, and would cause the fish to ascend ; 

 conversely, a depression of the head, or flexing the tail down- 

 ward, would enable the fish to sink. For so ponderous a body 

 as Dinichthys, a firm attachment and support for the muscu- 

 lature was essential, as well as means for holding the massive 

 plates in position; and these requirements could have been 

 abundantly supplied by the structure of the dorsal shield, 

 supplemented by its relation with the neural spines. 



Doubtless much light can be shed on these problems by a 

 study of the musculature in the recent Siluroids. The writer 

 is indebted to Dr. Samuel Garman for valuable information 

 regarding this group of fishes, and for the opportunity of dis- 

 secting an alcoholic specimen of Loricaria from South America. 

 The condition of the latter, however, although showing excel- 

 lent skeletal details, was unfavorable for an investigation of 

 the muscular system. 

 Museum of Comparative Zoology, Cambridge, Mass. 



