8 Trans. Acad. Sci. of St. Louis. 



and forming two openings on the interior side. In silicious 

 casts of this species these Httle openings are often found to 

 be filled with silicious matter, and will naturally show on the 

 summit of such casts a little bifurcating tube, as illustrated 

 by Meek and Worthen,* which undoubtedly gave origin to 

 the supposition that two of the hydrospiric tubes were united 

 near the summit into one, as figured by Billings f and Lud- 

 wig4 By a little reflection, however, it is easily compre- 

 hended that these bifurcating tubes have nothing to do with 

 the hydrospiric tubes and that it is only silicious matter 

 filling out these little channels, for they are never found on 

 casts of other species than those which have the deltoid 

 pieces pierced in the described manner. 



From the foregoing it will be seen that we have nearly 

 every degree of development in form and size represented, 

 and this gradual development of the lateral expansion as we 

 find it in the typical specimens of Pentremites sidcatus, pyri- 

 formis, fiorealis and others is a suflicient and good reason 

 to exclude the newl}^ rehabilitated genus Pentremitidia of 

 D'Orbigny, whose principal distinguishing character consists 

 in the absence of the lateral expansion, as plainly stated 

 by him : ' ' Calice compost de deux series de cinq pieux 

 superposees."§ His types for this genus were Pentre^nites 

 schultzii and paillettei. But the absence or non-develop- 

 ment of the lateral expansion, as it is often found in the 

 typical species, and always in the young specimen of the 

 same species, does not justify the creation of a new genus, as 

 was well remarked by Dr. Koemer: " Gewiss verdient es aber 

 keine Nachahmung." |1 Otherwise the young specimens of 

 Pentremites sulcatiis, pyrif or mis, etc., would belong to the 

 genus Pentremitidia during their juvenile state, and after- 

 wards when fully developed to Pentremites ; see Plate VI., Fig. 



♦ Illinois Geological Survey, Vol. V., plate IX., Fig. 2 c, p. 473. 



t Palaeozoic Fossils, Vol. XL, Part I., Fig. 60, p. 102. 



X Morphologische Studien an Echinoderraeu, Band I., p. 289, Plate 27, 

 Fig. 36 and 37. 



§ M. A. D'Orbigny. Prodrome de Pal^ontologie, Vol. I., p. 102, 



II Roemer. Monographic der fossilien Criaoidenfamilie der Blastoideen, 

 p. 49; and Wiegmann's Archiv fiir Naturgescliichte, Jahig. XVII., B. 1. 



