86 ECHINOIDEA. II. 



single plate 2 of the anterior interambulacra and by the simple pores. The figure of Offaster corculum 

 given by Loven (On Pourtalesia. p. 92) is highly interesting as showing the beginning of such an 

 arrangement in the antero-lateral interambulacra ; this form does undoubtedly show us the way from 

 the Ananchytidce to the Urechinidce — also the pores are very small and the ambulacral plates high 

 in this form, characters pointing towards the Urechinidce, but the pores are, however, double as in 

 the true Ananchytidce. 



The Pourtalesiidce evidently form another separate branch from the Ananchytidce^ with which 

 thev agree in having the second plate in the antero-lateral interambulacra paired. The main char- 

 acter of this family otherwise is the oral invagination of the anterior ambulacrum with the struc- 

 tural features of the actinal part of the test resulting therefrom , and the vertical position of the 

 peristome. The homoiopodous condition of the tube-feet can no longer be regarded as a family char- 

 acter, since Sternofiatagus is shown to have penicillate actinal tube-feet like the Urechinidce and An- 

 anchytidce; but the simple or even quite rudimentary pores afford another good distinguishing char- 

 acter between this family and the Ananchy tides, in which the pores are double. Whether we have to 

 seek the transitional forms between the Ananchytidce and the Pourtalesiidce in such forms as Infulaster, 

 Hagenowia or Stegaster I dare not have any definite opinion, being too little acquainted with these 

 genera; but as far as I can see it is rather probable. In any case the Urechinids cannot be regarded 

 as ancestral forms of the Pourtalesia; ; the single plate 2 in the antero-lateral interambulacra is alone 

 a sufficient proof that there cannot be a direct genetic connection between these two families. 



The genus Calymne cannot be referred to either of the two families named, differing from the 

 Urechinidce in having the plate 2 of the anterior interambulacra paired, from the Pourtalesiidce in 

 having no oral invagination and from the Ananchytidce in having simple pores. It must then, evidently, 

 form a separate family, Calymnidae. Whether the marginal fasciole is a family-character it is impos- 

 sible to decide, as long as this form is the only one known of the family; but judging from the other 

 families it will scarcely be more than a generic character. 



The genus Pilematechinus would be exceedingly interesting, in case the structure of its plastron 

 were really as figured and described by Agassiz in the Panamic Deep-Sea Echini ; it would then 

 be a living representative of the forms in which the plastron is still in the primitive condition, known 

 in the Collyritidce and Cassidulidce, and from which the meridosternous and amphisternous plastron 

 are later developments. Pilematechintis would then be the most primitive of the recent Spatangoidea. 

 It can, however, scarcely be doubted that Pilematechinus is a true meridosternous form, belonging to 

 the Urechinidce, the plate interpreted by Agassiz as the labrum being in fact two plates, a short 

 labrum followed by a larger sternum. -- A feature of great interest in Pilematechintis is that it has 

 comparatively well developed auricles; this evidently points towards the Guatliostoniata, viz. the Holcc- 



le labrum . This would involve the incorrectness of all the cases of Heteronomy in the Interambulacrum 1 in the Spatangidcc, 

 pointed out by Loven. Though I cannot follow Loven in all these instances, I think that in many of them Loven's inter- 

 pretation of the larger plates as being fused from two or three is quite correct. That the labrum is really a single plate I 

 most decidedly agree with Lambert, and I suppose that I am likewise in accordance with Lambert in rejecting his previous 

 view (Etudes morph. sur le plastron des Spataugides. p. 63, 72), that in the Spalaugida the labrum should be considered 

 comme une piece complexe formee par la soudure intime de divers elements empruntes aux deux series des assules con- 

 stitutives de rinterradium impair », viz. composed of the two (theoretical) plates a. 1 and b. 1 and further of the plates a. 2 

 and b. 2, tbe great sternal plates being thus really a. 3 and b. 3. 



