F. G. Percival—Punctation of Terebratulid Shells. 51 
suggests its use as a tactile organ. Although stich elongation is not 
uncommon among modern Teleostean fishes, it does not appear to have 
_ been observed hitherto among Mesozoic Ganoids. I may also add 
that some of the bones, such as the supraclavicle and the neural 
arches fused with their curved spines, are exact miniatures of some of 
the bones of the gigantic Leedsia problematica from the Oxford Clay. 
They therefore tend to support the opinion that this largest known 
Mesozoic Ganoid belongs to the Pachycormide. 
EXPLANATION OF PLATE II. 
Saurostomus esocinus, Agassiz; nearly complete fish, about one-eighth 
natural size. Upper Lias: Holzmaden, Wiirtemberg. British Museum 
No. P. 11126. Prepared by Mr. Bernhard Hauft. 
Il.—On tHe Poncrarion oF THE SHELLS or ZeRERRATULA. 
By F. G. PERCIVAL, B.Sc., F.G.S., Assistant Lecturer in Geology at the 
University of Manchester. 
(PLATE III.) 
ile 1844 Carpenter [1] divided the fossils then known as Zerebratula 
into two groups—a perforate group, having the test covered with 
’ minute pores, and an imperforate group (the Rhynchonellids). These 
perforations (see Plate III) correspond to tubular processes of the 
mantle. 
Sharpe [2] suggested that these cecal processes of the mantle had 
a respiratory function; but the shell is covered with a chitinous 
periostracum which is imperforate. Carpenter considered this might 
be cellular, allowing water to pass through. Sollas [3] in 1885 
suggested that they were sense organs affected by light, since the 
- periostracum is transparent. Morse [4] offers the suggestion that 
they are organs of general sensibility. 
The tubules are not of uniform diameter throughout. The inner 
half, near the mantle, according to King [5] is narrow, the outer 
half wider, with a sudden dilation at the ‘mouth’. From the mouth 
a number of fine lines or tubes radiate. It is interesting to note 
that from the outer lip of each cecal process a number of fine cilia 
radiate. Possibly these cilia fit into the radiating tubes (see Morse 
[4]). The radiating tubes are rarely seen in fossils. An example is 
shown in Pl. III, Fig. 3. 
In some forms, e.g. 7. punctata, Sow., the puncte are elongated 
and slit-like at the outer surface, but have a round cross-section 
a little farther in. This is well shown in Fig. 5, where part of the 
outer layer has flaked off. 
In Zerebratulina (Fig. 1) the cecal tubules branch as they pass 
outwards, but I have not observed this in any species of Zerebratula. 
It would, however, be difficult to recognize in fossil specimens, even 
if present. 
The examination of the puncte, or external openings of the tubules, 
was first undertaken ‘with the hope of using their variations as aids 
in distinguishing species. The puncte are arranged in rows roughly 
parallel with the growth-lines. When the apertures are slit-like 
the long axis of each slit points to the umbo, and this emphasizes the 
fact that they are also arranged in rows radiating out from the umbo. 
