Erroneous Echinodermal Identijication. 105 



18 mm., tlie ambulacrum 9 mm., and tlie poriferous zone 

 1'5 mm. The tubercles are nearly all of the same size, and 

 arranged in transverse rows occupying the entire widtii of 

 the plate, usually a single row in each plate, and are pinkish 

 orange in colour. 





-A 



x^.. 



Pseudechinus huttoni. 



Fig. 1.— Portions of ambulacrum and interambulacruni, showintr arranpe- 

 ment of tubercles at the ambitus (A) and immediately above 

 X 2. 



Fig. 2. — An ambulacval and an iuterambulacral plat*, x 4. 



The form is more ov less globular, with a slight tendency 

 for the apical region to be subconically elevated; at the same 

 time T have specimens which are distinctly depressed, so as 

 to be somewhat bun-shaped. 



Inter ambulacrum. — In each plate tlie tubercle situated in 

 the middle of the plate is rather larger than the rest, and 

 may be termed a primary ; on each side of this are three 

 slightly smaller ones, which may be termed secondaries. In 

 some instances a fourth secondary occurs near the external 

 (ambulacral) margin — so that at the ambitus there are usually 

 seven tubercles in a row, less usually eight. 



In about every alternate plate this row is duplicated on 

 the ambulacral side of the primary. 



Near the abactinal region the number of tubercles decreases 

 rather suddenly : the upper five or six jjlates bearing less than 

 seven ; the topmost having only one or two secondaries in 

 addition to the primary. 



