444 Deep-sea Trawling off the S.W. Coast of Ireland: 
is distinguished from the commoner form he enumerates the 
spines, S. purpureus having “ radiolis primariis eminentiori- 
bus colore albicante insignibus ;”’ but the differences between 
the two species in this particularare much reduced when a series 
is examined. On the other hand, whether specific characters 
are useful or not, spines are certainly valuable to the indi- 
vidual which possesses them. As the accompanying measure- 
ments show, the form of S. Raschi may vary a good deal, and 
these variations must atfect such characters as are indicated 
by such expressions as “ ambitu fere orbiculato, dorso multo 
minus convexo, margine magis rotundato.”’ This brings us 
to another still unsettled question :—How far are characters 
that vary within considerable limits to be used as specific 
characters ? and to such a question we can well imagine 
different systematists giving very different answers. 
(Questions like these may well be raised, if the answers 
that are given are tentative and not dogmatic. The only 
moral [ can definitely see is one which has been, but must 
again and again be, insisted on. ‘The definitions of species are 
often drawn up from a few specimens, or perhaps only one ; 
with increased knowledge of the representatives of such species 
our judgment as to its “characters is bound to be affected by 
the variations whieh will undoubtedly present themselves— 
very much so when the describer has a small knowledge of 
the group—to some extent even when the description is by 
the hand of a master in his science. 
Among the specimens is one which is considerably depressed 
and deformed; but the abnormal characters which it presents 
do not seem to throw any light on the characters of the species. 
Measurements of Spatangus Raschi. 
Percentage value of 
oo  ( 
a 
Long. diam. Transy. diam. Height. 
107 86 60°74 
91 85:7 70°53 
90 97°7 755 
Brissopsis lyrifera, Forbes. 
Two spineless specimens, of ordinary size, were taken in 
5 fath. 
V. HOLOTHURIOIDEA. 
Holothuria tremula, Gunner. 
Dredged at 100 and 315 fath. ; it was dredged from greater 
depths than these by the Norwegian North-Sea Expedition. 
