INTRODUCTION. 31 



present day. The very numerous Starfishes (more than 4UU in an exposure of 

 200 square feet) were found intimately associated Avith clams, (J rammysia and 

 Fterinea, in such a way as to suggest that they were feeding on the clams at the 

 time they were buried in the strata. 



The report states that "nearly every Grammysia or Pterinea found in this 

 layer has a star in or on it, sometimes several about its edges in attitudes 

 suggestive of attack, and it is altogether reasonable to believe that the hostility 

 between the starfish and the bivalves had fully developed at this early day in the 

 history of the earth." 



The Transition to the Ophiuroidea. 



Tlie Mouth-Frame of Stenaster obtusus. — It has been shoAvn that the structure of 

 the arm of Stenaster obtusus shows an early stage in the advance towards the 

 Ophiuroidea. Similarly the mouth-frame of this species helps us to nnderstand 

 the origin of the modifications found in recent forms of the class. 



An oral view is shown in PI. I, fig. 6. Judged from this aspect there is 

 little advance from Eoactis simplecV. Neither the ambulacralia nor the adam- 

 bulacralia of the mouth-region are much differentiated with respect to size, and 

 correspondingly there could have been little adaptation of the tube-feet to form the 

 large buccal tentacles of the true Ophiuroidea. 



Apically also the structure at first sight appears simple. If we examine more 

 closely, however (PI. I, fig. 7), we find that the first adambulacralia are reduced, 

 and consequently not seen from the dorsal surface. Both the first and second 

 tube-feet, as in Ophiuroids, are supplied by a single vessel which comes off from 

 the ring canal, and pierces the first ambulacral (t',2) in figure). An exceedingly 

 well-preserved specimen in my possession shows the course of this vessel (Text- 

 fig. 28). A forwardly projecting branch runs in a groove towards the depres- 

 sion for the first tube-foot, and a very short downwardly projecting branch pierces 

 the first ambulacral and supplies the second tube-foot. A comparison of this 

 structure with that shown in Text-fig. 27 is most instructive as showino- the 

 forward shift of the second bnccal tentacle in Recent Ophiuroidea and the conse- 

 quent elongation of its water-vascular canal. 



Besides these resemblances to the Recent Ophiuroidea, Stenaster shows 

 pecnliarities which are found in other Palseozoic species and serve to show their 

 digression from the main stock. There are a comparatively large number of 

 ambulacralia intimately connected with the mouth-frame. In Recent Ophiuroidea, 

 it will be remembered, the " jaws " are formed from fused mouth-angle plates and 

 (?!, while ^2 are free vertebrae. In Stenaster a^ often fuses with Oo. Further, the 

 bases of these ossicles are not approximate as in Recent Ophiuroidea, biit widely 

 separate (compare PI. I, figs. 3, 6 and 10). 



