86 



ECHINOIDEA. II. 



siiij»-leplate 2 of the anterior interarabulacra and b\- the simple pores. The figure of Offastcr coraihun 

 given by Loven (On Ponrtale.sia. 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 wa\- from 

 the AiKutchytidcc to the Urcchinidm — also tlie pores are very small and the ambulacral plates high 

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

 the true Ananchyt/dte. 



The Pour talcs iida- evidently form another separate branch from the Ananchytidcc^ with which 

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

 acter of this familv 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 homoiopodons condition of the tube-feet can no longer be regarded as a family char- 

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

 anchytidcr : but the simple or even (|uite rudimentary pores afford another good distinguishing char- 

 acter between this famih- and the Aiiaiichytidir. in which the jiores are double. Whether we ha\e to 

 seek the transitional forms between the Ai/aiic/iytidcc and the Ponr/fi/rsiidic in such forms as ////«Am/c/-, 

 Hagcnoivia or S/igastcr I dare not have anv definite opinion, being loo 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 Pourtalesise ; 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 Calyiniic cannot be referred to either of the two families named, differing from the 

 Urcchinida in having the plate 2 of the anterior interambulacra paired, from the Pourtalesiida in 

 haviu"- no oral invagination and fruui i\\K: AiKuicliytida- in having simple jiores. It m\ist then, e\'i(leull\-, 

 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 Pilcmatcchiinis 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 Collyritidw and Cassidulidw , and from which the meridosternous and amphisternons plastron 

 are later developments. Pilctnatcchinus woidd then be the most primitive of the recent Spataiigoidca. 

 It can, however, scarcely be doubted that J'ilcuiahc/iiinis is a true meridosternous form, belonging to 

 the Urcchinidm, the plate interpreted by Agassiz as the labruni being in fad two plate.s, a short 

 labrum followed by a larger .sternum. - A featme of great interest in Pihiiitilrcliiinis is that it lias 

 comparativel\' well developed auricles; this evidenth- points towards the (iiKithosloiiKihi, \iz. the /AV^'c- 



Ic labrum ■. This would involve the incorrectness of all the cases of Heteronoinj in tin.: Interanibulacruui 1 in the Spaiangida:, 

 pointed out by L,ov6n. Though I cannot follow Loven in all these instances, I think that in many of them Lov^n'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 (Ivtudes morph. sur le ]>lastron des Spatangides. p. 63, 72), that in the Spatangida: the labrum should be considered 

 • comme uiie pii-ce complexe formee par la soudure intime de divers elements empruntfe aux deux scries des assules con- 

 stitutives de I'interradium impair , viz. composed of the two (theoretical) plates a. i and b. i and further of the plates a. 2 

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



