POSITION OF APPENDAGES. 75 



this way, on the tips of the toes, so to speak. In such the limbs would probably extend 

 downward and outward, with the flattened sides vertical. 



The limb of Triarthrus, however, is of another type. The endopodites are long, slender, 

 flexibly jointed, the whole endopodite probably too flexible to be used as a unit as a leg 

 must be in walking on the "toes." The proximal segments of" the thoracic and pygidial 

 endopodites are, however, triangular instead of straight-sided, and, the spine-bearing apex 

 of the triangle being ventral, it enabled the endopodites to get a grip on the bottom and 

 thus push the animal forward. This method of progression was more clumsy and less rapid 

 than that of Ncolcnus, but it sufficed. The natural position of the endopodite when used 

 in this way would seem to be with the flattened sides of the segments standing at an angle 

 of 30 to 45 with the vertical, thus allowing a good purchase on the bottom and at the 

 same time offering the minimum resistance to the water when moving the appendages 

 forward. 



Isotelus has endopodites different from those of either Neolenus or Triarthrus. They 

 are composed of cylindrical segments, the joints indicating a certain amount of flexibility. 

 Since there is no method by which the segments may get a purchase on the bottom other 

 than by pushing with the distal ends, it would seem at first thought that Isotelus, like Neo- 

 lenus, crawled on its "toes." The endopodites of Isotelus are however, short and feeble 

 when compared with the size of the test, while the endobases of the coxopodites are ex- 

 traordinarily developed. These facts, together with certain trails, strongly suggest the use 

 of the coxopodites as the primary ambulatory organs, the endopodites probably assisting. 

 In this event, the position of the endopodites and coxopodites would be downward, both 

 outward and inward from the point of attachment, and the motion both backward and 

 forward. The fact that in the specimens as preserved the coxopodites point backward and 

 the endopodites forward indicates that the limb as a whole swung on a pivot at the appen- 

 difer. It is of course natural to suggest that the coxopodites and endopodites of all the 

 trilobites with wide axial lobes, Nileus, Bumastus, Homalonotus, etc., were developed in 

 this same way. 



Cryptolithus presents still another and very peculiar development of the endopodites 

 where ability to get purchase on the sea floor is obtained by a stout limb of slight flexibility, 

 bowed and turned backward in the middle, where an enlarged segment insures stiffness. 

 The segments are flattened, and since the greatest strength when used in pushing and crawl- 

 ing is in the long axis of the oval section of the flattened limb, it seems probable that these 

 limbs did not hang directly down, with their sides vertical, but that their position in life 

 was very much the same as that in which they are preserved as fossils. By moving these 

 bowed legs forward and backward in a plane at a small angle to the surface of the body, a 

 powerful pushing impetus could be obtained. They may, however, have occupied much the 

 same position as do those of Limulus. 



In the case of the endopodites, therefore, it is necessary to study the structure and prob- 

 able method of their use in each individual genus before suggesting what was the probable 

 position in life. In the act of swimming, the position was probably more uniform. When 

 the endopodites were used in swimming, as they undoubtedly could be with more or less 

 effect in all the trilobites now known, those with flattened surfaces probably had them at 

 such an angle as to give the best push against the water on the back stroke, while on the 

 forward stroke the appendage would be turned so that' the thin edge opposed the water. 

 The great flexibility of attachment would certainly permit this, though unfortunately nothing 



