80 CALAMOCRINUS DIOMED.E. 



seem justified in looking upon the mode of formation of the plates of the 

 anal system of the Salenidee and of the Echinidse proper as an atrophism 

 of the primary mode of formation of these plates in the earliest known 

 Sea-urchins. Only in the Echinida3 proper the plates of the anal system 

 are not as it were separated into two categories, as in the Salenida3, 

 one of which seems to form a part of the genital ring (see Figures quoted 

 above), while the other plates correspond to the smaller more or less irreg- 

 ularly arranged plates of the anal system in the Echinidae. See the Figures 

 of Echinus horridus (Plate XXXI. Fig. 11), of Toxopneustes varlegatus 

 (Figs. 8-10), and of Temnopleurus Eeynaudi (Fig. 2). 



Tiarechinus does not seem to me to have the great importance assigned 

 to it by Loven in tracing the homology of the calyx of Echinoids with 

 that of the Crinoids. Judging merely from the description of the genus 

 given by Neumayer* and Loven,t I am more inclined to look upon this 

 remarkable Sea-urchin as an eminently embryonic type, but possessing at 

 the same time the unique character of having three rows of interambidacral 

 plates, — a structural feature which it possesses in common with some of 

 the Palaschinidae, where there are five and seven interambidacral rows. 

 Tiarechinus is closely allied to the earliest fossil representatives of the 

 ArbaciadfB. The structure of the apical system is to my mind eminently 

 Arbacia-like, and the spreading of the ambitus seems to be the first trace 

 of the peculiar petaloid condition we find in the Arbaciadae and Echino- 

 metradoe. Unfortunately, the anal jjlates are not preserved. To judge 

 from analog}^, I am inclined to look for a pyramid of four or five anal 

 plates, as in the Arbaciadfe. No one can fail to see in the solid undi- 

 vided apex a condition of things very similar to that pi-esented in a 3'oung 

 Arbacia such as I have figured in the Revision of the Echini, page 734. 



The splitting of a tubercle in the suture of the interambulacral area is 

 not a feature luiknown among Echini, as Neumayer states. It is of com- 

 mon occurrence in the plates of the ambulacral system in the regular 

 Echini (Desmosticha), and at the junction of the plates of the ambulacral 

 and interambulacral systems, and is quite common in both the ambulacral 

 and interambulacral areas in the Clypeastroids and especially the Spatan- 

 goids (Petalosticha). 



that has thus far been observed regarding the development of the plates of the anal system. Existing 

 anal plates may break up into a larger number, or additional plates may be formed by intercalation 

 between the older ones, but no case has been observed in which they have co.alesced. 



* Morpholog. Studien an Echinodermen. ■(• Pourtalesia. 



