MONOGRAPH OF THE EXISTING CRINOIDS. 



85 



the water. It is because of this that lime-depositing organisms are more abundant 

 in the warmer than in the colder ^Yaters. and that the lime deposited by organisms 

 in the warmer regions is incomparably in excess of that laid down in colder. 



Promachocrinus and Thaumafocrlnus both inhabit water which is cold, abnor- 

 mally cold for species of their respective groups, yet abounding in available food, 

 and 'it may be that, through the relative difficulty in extracting the necessary 

 lime from solution in the regions where they live, the formation of the skeleton 

 can not keep pace with the growth of the ^dsceral mass, this tending to render 

 more difficult the closure of the radial circlet and permitting, or even inducing, 

 the formation of the interradials upon which the interradial postradial series 

 eventually are formed. It is worthy of note in this connection that the related 

 genera Solanometra and Aathometra which occur with Promachocrinus, as well 

 as other unrelated genera, exhibit marked features which may be logically 

 attributed to cold. In Solanometra all the skeletal elements are abnormally short, 

 just as in many other species occurring in water abnormally cold for their groups, 

 in Coviatiilella IracMolata (part 1, fig. 77. p. 130, and pi. 50, fig. 1332) and in 

 Antedon petasus (part 1, fig. 103, p. 165; compare with figs. 105, p. 169, and 106, 

 p. 171), for example, and in Anthometra the radials and postradial series remain 

 spinous' and carinate, never assuming the mature smooth form. Florometra comes 

 nearest to the normal type from which ProniachocHnus, Solanometra, and An- 

 thometra are derivatives. 



The number of postradial ossicles or brachials in each linear series beyond the 

 outermost axillary is usually from 150 to 180, though in certain genera, such for 

 instance as Asterometra (figs. 206. 207, 209, pp. 143, 145. 149, and part 1. fig. 94, p. 

 155) and PtUometra (fig. 204, p. 139. pi. 53, fig. 1346, and part 1, fig. 93, p. 153 , 

 there may be as few as 70. and in certain others, such as Helwmetra (fig. 233. p. 193) , 

 Florometra, and the larger comasterids as many as 270, or even nearly 300. In 

 the fossil Vintacrinus, which has excessively elongated arms, the number is vastly 

 <rreater. P. H. Carpenter counted 140 in an arm of Antedmv hi-fida 4 inches long. 

 The shape of the brachials, as seen in a dorsal view, varies from triangular 

 with the lencrth equal to the width through wedge-shaped to oblong with a width 

 of many times the length, or with a length of many times the width and more 

 or less strongly incurved sides. , • , • v 



The first syzygial pair, composed of the third and fourth brachials, is ordi- 

 narily rectangulaV'; occasionally, however, it is more or less wedge shaped and, 

 when this is^the case, the inner side is longer than the outer. Only in rare 

 instances, as in AtelecrinMS (part 1. figs. 123, p. 192, and 124, 125, p. 193), is the 

 additional inner length distributed equally on the hypozygal and on the epizygal. 

 Usually it is confined mostly, or even entirely, to the epizygal, which becomes 

 very obliquely wedge shaped or triangular, the hypozygal remaining nearly or 



^^^ Typicatiy in 10-armed forms the anterior sides of the axillaries are slightly 

 concave and the two chords of these concave sides meet at the anterior angle in 

 such a way that this measures about 90°. In certain cases, usually correlated with 

 a similar production of the interradial angles of the radials, the anterior angle 



