^®1' 55-] SILURIAN ECHINOIDEA. AND OPHIUKOIDBA. 703 



region. The largest example measures 5 cm. in diameter. In all 

 cases the organism that furnished the cast has been compressed 

 from above downward, never from side to side. A careful study- 

 enables me to supplement the excellent descriptions of Salter and 

 Wyville Thomson in several important particulars. The plates of 

 the test are all of them overlapping ; the wide interambulacra 

 become narrower towards the mouth, and cease at the peristomal 

 margin, or are continued only a very short distance beyond by a few 

 minute plates. The series of adambulacral plates is fairly regular, 

 and the succeeding rows of the interambulacra maintain a rough 

 parallelism with it. The ambulacral areas do not cease at the 

 peristome, but are continued as far as the pointed ends of the jaws, 

 the sides of adjacent ambulacra lying in contact immediately beyond 

 the termination of the interambulacra (fig. 6). 



Dr. Gregory, after a comprehensive survey of the characters 

 which have been regarded as diagnostic of the Echiuoids, selected 

 the disposition of the ambulacral plates as a decisive criterion 

 in the case of Falceodiscus, and, asserting that these lie flush with 

 those of the interradii, pronounced in favour of its relationship to the 

 Echinoidea : a conclusion doubtless correct, but deduced from false 

 premises, for the ambulacra do not possess the characters assigned 

 to them, and it is to the buccal apparatus that we must turn for 

 convincing evidence of the true nature of PalcEodiscus. 



The buccal armature (fig. 7). — The ventral surface seen from 

 within affords an excellent study of the 'lantern.' Most con- 

 spicuous are five strong rectangular bars with rounded corners 

 situated radially ; on either side of each of these a strong ridge is 

 seen, which widens out flange-like distally. If these ridges represent 

 the ' epiphyses ' of the alveoli, then the rectangular bars can 

 scarcely be other than the intermediate piece or ' rotula.' The 

 epiphyses do not meet interradially as in recent urchins. In 

 addition to these elements a third is sometimes met with which 

 presents a strap-like form, bifurcate at the distal end. If this should 

 be the ' radius,' as I believe it to be, it must have suffered some 

 displacement, since it is never seen lying immediately over the middle 

 line of a rotula. 



In addition to these elements, teeth of Echinoid character are 

 present ; traces of them may 



still be seen in connexion Fig. 8. — An isolated tooth of 



with the ' lantern,' and in Palseodiscus ferox ( x 9). 



one instance an isolated . - 



tooth (fig. 8), freed from its ^: _ .^ 



sheath, lies fully exposed ~ — — ^ 



over the surface o£ an 



interambulacrum. It is composed of denser material than the other 

 ossicles of the skeleton, and in consequence appears less opaque ; 

 it has an elongated lancet-like form, is sharp at one extremity, 

 longitudinally grooved, and divided at the other extremity by a 

 single furrow ; its length is 5-5 mm., its maximum breadth 0*2 mm. 



The close relation of Falceodiscus to the Echinoidea being thus 



