222 Messrs. R. Etheridge, Jun., and P. H. Carpenter on 



]:)ropose tliat this name sliould be) to such species as P. Godoni, 

 Defrance, sp., P. sulcatus, Romer, and P. jyy^'^formis^ Say, &c. 



Tlie variability of external form is perhaps as marked in 

 this genus as in any member of tlie Blastoidea. Two distinct 

 types are met with, the pyriform and chavate. Starting with 

 the type species, P. PaiUettl^ representing the former, we liave 

 in the one direction a gradual transition through P. lusita- 

 nica to the mucli more elongated calyx of P. Schultziiy with 

 its truncated summit and expanded ambulacra. In the other 

 direction we observe a gradual change in outline through 

 P. eifelensis to P. clavata and P. acatangula^ and so on to 

 the Orophocrinoid species P. angulata and P. similis. 



The amount of truncation of the summit also varies consi- 

 derably. In P. Hclmltzii we see the limit of variability, the 

 summit being broad, flattened, and extending across the wliole 

 widtli of the calyx. The width of the summit is decidedly 

 less in P. Pailletti] again sunxUer in P. lusitanica, and still 

 more so in the aberrant P. angnhda and P. similis. Similarly, 

 the broader the summit the wider tlie ambulacra ; hence we 

 meet with tlie greatest development of this kind in P. Sclmltzii, 

 the ambulacra gradually becoming narrower in P. lusitanica 

 and P. Paillettij and reaching the limit in the same two forms 

 mentioned above. 



As regards the more intimate structure of the ambulacra, 

 we may describe three exam])les. In the first of these, P. 

 ScJndtzii, the ambulacra are very wide, flattened, or a little 

 concave, and more or less deltoid in form. The lancet-plate 

 elongatcly petaloid, more markedly so than in most Blastoids. 

 The side plates abutting against it are fourteen on each side, 

 narrow, and oblong. The outer side plates are very small, 

 placed quite at the extremities of the side plates, and nearly 

 on the same level with thein. In the clavate and pyriform 

 types the structure is more or less similar, as, for instance, in 

 P. clavata and P. lusitanica. The side plates do not reach 

 the edges of the radial forks ; but the intervening spaces are 

 bridged over by the outer side plates, alternating with the 

 pores, which are very large. 



We are acquainted with the hydrosjiircs of only two species, 

 P. Hchidlzii and P. Pail/ctti, in both of wliich these organs are 

 eight in uuinbcr on each side of the ambulacra. 



One of the most important features in Pentremitidea is 

 given by the oral plates, which, in consequence of their very 

 small size and close connexion with the summit, afford one of 

 the most stable characters of the genus. In most of the 

 species they arc quite inconspicuous, and cannot be distin- 

 guished except as formhig the dividing septum of the spira- 



