PENTAMERUS. 121 
in the Museum of Practical Geology, 8 in the British Museum, and 2 in the 
Geological Society’s Museum. 
Remarks.—From the great variability of this abundant species and the distorted 
condition in which it often occurs, its detached valves are often the source of great 
perplexity. 
Davidson has shown that it is distinct from P. galeatus, Dalman, with which it 
had been united by Kayser. 
The minute surface-ornament is sometimes clearly seen, and the differences in 
its character are somewhat puzzling. Many specimens are covered by numbers of 
microscopic triangular nodules arranged more or less regularly in arching lines, 
and so distinctive as to permit fragmentary specimens to be recognised by them. 
It appears to me that the ornament of the shell described by Maurer as Allorisma 
cancellatum is so similar that it may probably be only a distorted dorsal valve of the 
present species, especially as I have often found great difficulty in distinguishing 
similar valves at Lummaton from Lamellibranchs. 
On the other hand, a few very large and compressed specimens in my 
Collection, which were identified by Davidson, show no signs of this minute 
tuberculation, but instead are covered in the marginal parts by almost equally 
fine, sharp, and regular transverse elevated lines. It seems difficult not to regard 
these as surface-ornaments, or to imagine them covered by pustules. Again, a 
similarly shaped shell has equally fine radiating lines, but in this case it is easy to 
reconcile them with other markings, as the surface is more worn. 
Two large specimens are peculiar from their great depth and narrowness and 
the regularity and elongation of their narrow dorsal fold, and might perhaps, if 
more such specimens were found, be advantageously separated as a named variety. 
P. optatus, Barrande, as given by Schnur,' is very like a variety of the present 
species, but Barrande’s own figures show that it is distinguished by its more 
inflated sides. 
A shell in Mr. Vicary’s Collection from Wolborough, labelled by Salter 
“ Spirifer new,” has caused much perplexity, but it must, I think, really be an 
abnormal or distorted variety of P. brevirostris. I think the ridge which made 
Salter regard it as a Spirifer is accidental, and that it has really no true area. Its 
dorsal valve has acentral angular depression or sinus, and might without difficulty 
be supposed to be a valve of this species, but there is a similar central sinus as 
well as a lateral one in the ventral valve. These marks, however, do not appear 
to be natural, but due to distortion in growth. There are signs of a strong 
median septum in the ventral valve and two internal processes in the dorsal valve, 
as in P. brevirostris. It almost exactly agrees with Bronn’s figure of Sp. glaber. 
1 1854, Schnur, ‘ Paleontographica,’ vol. iii, p. 196, pl. xxxii, figs. la, b. 
VOL. II. 16 
