Ctassificatory Position of Hemiaster elongatus. 231 



and its posterior end reaches backwards almost quite behind 

 plate 3 of row " Z>," to reach and touch the plate " a " 5 

 "which is almost in the median line, having both its lateral 

 sutures in contact with ambulacral plates (that is, with the 

 row " h " of amb. V. and row '^ a " amb. I.). Plate 2 of row 

 " h " is much shorter than the corresponding plate of row 

 " o," and it only extends as far back as the posterior edge of 

 the third ambulacral plate of row "a," — ambulacrum No. I., 

 or the right postero-lateral. The anterior suture of plate No. 2, 

 row " ^>" of iuterradium 5, is rather wide, and is in contact 

 with nearly the whole of the posterior suture of the first plate 

 of that interradiura, and thus the anterior suture of plate 2, 

 row " a," is pushed so far to the true left that a very small 

 portion of the plate comes in contact with plate No. 1 . 



The posterior suture of the short plate £, row "J," is 

 broad and almost forms a right angle with the median suture 

 of the iuterradium ; it is in complete union with the anterior 

 suture of plate J, row "■ b." The outer margin of this short 

 plate is in contact with plates 2 and 3 of row " a " of ambu- 

 lacrum No. I. only. 



Plated of row " b " of the odd interradiura is irregularly pen- 

 tagonal in shape; it is about tliree fourths the size of the plate in 

 front of it, is larger tlian the corresponding plate 3 of row " a," 

 and is placed altogether anteriorly to this last. Moreover it 

 extends from right to left, over the direction of the median 

 line, and is in contact with no less than five plates. Tiie 

 anterior suture is entirely attached to the interradial plate 2^ 

 row '^ b ;" the left-hand suture is in contact with the posterior 

 riglit suture of plate 2, row " a ;" the right-hand suture comes 

 against the fourth and fifth plates of the ambulacrum No. I., 

 row "a;" the left posterior suture joins enth-ely with the 

 right antero- lateral suture of plate 3, row " a ;" and the re- 

 maining suture, the right posterior, unites with the fifth 

 ambulacral plate of I., row " a." The plate now under con- 

 sideration seems, as it were, to compensate for the shortness 

 of the plate 2 of row " ^," iuterradium 5. In Loven's last 

 work on PourtaUsia &c. pi. xvi. fig. 185, there is a most 

 beautiful drawing of Palceostoma mirabile. It is of the actinal 

 part of a half-grown individual, and the sutures of the plates 

 are very visible. It shows the great size of the ambulacral 

 plates in relation to the interradials, and it indicates all that 

 has been stated above about the different plates and sutures. 

 Nothing can be more clear than the pentagonal shape of the 

 mouth and the extremely small part played by the ambulacra 

 in forming the margin. In fact, hardly any part of the 

 margin that relates to the interradia Nos. 4 and 1 is produced 



