152 CATALOGUE OF THE BLASTOIDEA. 



pyriform in shape, partially excavated in the substance of the deltoid plates, and 

 otherwise bounded by the lancet-plate and proximal side plates. Posterior spiracles 

 confluent with the anus, the combined aperture being somewhat larger than the other 

 spiracles Peristome of variable size, covered by a small dome of minute polygonal 

 plates, which may also cover the spiracles l , and is continued outwards as alternating 

 covering plates above the ambulacral grooves. Column round, and composed of thin 

 discoidal joints. Ornament consisting of fine stria? arranged parallel to the margins 

 of the plates. 



History. If we omit for the present the genera Eleutherocrinus, Lyon & Casseday, 

 Astrocrinus, T. & T. Austin, and Pent ephy Hum, Haughton, the history of the genus 

 Pentremites, as formerly understood, is practically that of nearly all the genera 

 which we shall subsequently describe. We shall, however, confine our historical 

 remarks to that group of species for which we restricted the name in 1882. 



The genus was originally proposed by Say in 1820, when, in addition to the general 

 characters of the calyx, he described the structure of the ambulacra, and said that 

 each of the apertures which we now call the spiracles " is the common aperture of 

 two tubes." He further supposed that " tentacula " (/. e. tube-feet) were protruded 

 " through the pores of the Ambulacrse," as is the case in the Echinoidea. 



In 1835' Dr. G. Troost 2 described the component parts of the ambulacra and the 

 construction of the ambulacral or hydrospire-pores in various species of Pentremites ; 

 and he noticed that each of the spiracular openings is often " divided into two parts 

 by a septum." 



The existence of the ambulacral appendages or " pinnulee " was noticed independ- 

 ently by Dr. F. Roemer and Prof. L. P. Yandell, in 1848 3 , in specimens of Pentremites 

 pyriformis and P. Godoni. Messrs. Owen and Shumard 4 have also described the 

 pinnules of P. Godoni, and a conical integument of small plates closing the five 

 summit apertures or spiracles. Mr. S. S. Lyon published some interesting remarks 

 in 1857, in which he endeavoured to prove the existence of "supplementary" basal 

 plates in the Pentremite calyx, underlying those usually called basals. This view has 

 been adopted by some Palaeontologists, and rejected by others, including ourselves 

 (see pp. 18-22). 



Dr. F. B. Shumard published some additional important observations on the 

 summit structure of Pentremites in 1858 5 , in which he showed that the mouth and 

 spiracles of P. conoidens, Hall, and P. sulcatus, Roemer, were covered in the one 

 case by six small pentagonal plates over each opening, whilst iu the latter the 



1 < )r each aperture may be separately closed by six small pentagonal plates according to Sliumard. 



2 Trans. Geol. Soc. Pennsylv. 1S35, vol. i. p. 227. 



3 Bull. Soc. Geol. France, 1848, tome v. p. 296. 



4 Report Geol. Survey, "Wisconsin, Iowa, and Minnesota, 1852, p. 592. 



5 Trans. St. Louis Acad. Sci. 1858, vol. i. no. 2, p. 243. 



