72 REPORT—1843. 
the calcareous part of the shell, composed of minute spiculz of carbonate of lime, 
which crumble under the finger. Cellular structure is found in all shells belonging to 
the order Margaritaceze. With these the Pinna should be placed, since the Mytilacee, 
with which it has been hitherto classed, possess a different structure. Deshayes, in 
the new edition of Lamarck, upon a comparison of the characters of the genus Pinna, 
considers it more nearly allied to Avicula than Mytilus. In the fossil genus Inoce- 
ramus, traces of the cellular membrane may be discovered by dissolving the shell in 
weak acid. ‘This structure also forms a thin layer between the epidermis and the 
nacre of Unionidz ; and also exists in the Ostracee. Dr. Carpenter has not found it 
in any shell not belonging to these natural families, except the anomalous genus Pan- 
dora. The second class of shells described by the author includes those possessing the 
Membranous Structure. In these the calcareous matter is deposited in laminz, sepa- 
rated by excessively thin membrane, which forms, in fact, a secreting surface. This 
membrane does not lie flat, but is usually extremely corrugated and folded, and these 
folds being repeated ina regular manner, give rise to the nacreous lustre of the shells. 
Fragments of the Haliotis, after being laid in acid for a week, are still nacreous. A 
similar structure is also found in the Cowry and other porcellaneous shells, which are 
composed of three layers, the direction of the folds being different in each layer, as 
seen in the fracture of the shell. Another peculiarity in the internal structure of some 
shells consists of minute tubes, ramifying copiously over the different layers of mem- 
brane, and sending branches into the adjoining laminze. Both the membranous and 
tubular structure are found in the Avicula cygnipes of the lias, showing that it belongs 
to the natural order Pectinide, and not to the Margaritacee. Prof. Phillips was 
aware, at the time he published this species, that it was nearly allied to the genus 
Lima and other Pectinide. ‘The structure of the Brachiopoda is peculiarly plicated ; 
and in the true Terebratulz all the laminations are perforated by minute holes, going 
quite through the shell. In a transverse section these perforations are seen to enlarge 
as they approach the inner surface, forming funnel-shaped openings, lined by a mem- 
branous prolongation of the mantle, which is closely adherent to the inner surface 
of the shell. In one recent species, the Terebratula psittacea, the perforations are 
wanting; but in this species the structure of the hinge is also different, bringing it near 
a particular division of the fossil Brachiopoda (Atrypa, Dalman). Most of the fossil 
Terebratule that are deeply plicated also want the perforations, and these probably 
constitute a separate group. In concluding, Dr. Carpenter pointed out some of the 
peculiarities in the structure of the skeletons of the Echinodermata, which have been 
noticed by Valentin in his Monograph. The calcareous plates of an Echinus consist 
of numerous thin Jamine, connected by little pillars; and as the laminz are per- 
forated all over, whichever way a section is taken it exhibits a minute net-work. Dr. 
Carpenter stated that before becoming acquainted with the observations of Valentin, 
which are confined to the genus Echinus, he had detected a corresponding structure 
in all the families of the class. Dr. Carpenter also exhibited diagrams of several 
beautiful forms of structure in the spines of different species of Cidaris, and in the 
plates, forming the column of several fossil and recent Pentacrinites, showing that a 
different arrangement of the internal particles obtained in every species. ‘This struc- 
ture is well preserved in the fossil species, although they possess a regular crystalline 
cleavage. 
On the addition of the order Nucleobranchia to the British Molluscous Fauna. 
By Professor E. Forses. 
Four years ago the author noticed minute needle-shaped animals floating under the 
surface of the water of the Frith of Forth and of the British Channel, which, when exa- 
mined under the microscope, presented several anomalous characters, which rendered 
their true position in the animal series doubtful. At the time he was able to make 
but an imperfect examination of them, which he laid before the Wernerian Society. He 
has since met with much larger animals, belonging to the same genus, inhabiting the 
Mediterranean sea, which enabled him to refer them to the genus Sagitta of Quoy 
and Gaimard, placed by those naturalists in the immediate neighbourhood of Firola, in 
the order Nucleobranchia. Both the British and Mediterranean animals have linear- 
lanceolate bodies, terminated anteriorly by an oval head, and posteriorly by a horizontal 
tail. Along the sides are two pair of fins, also placed horizontally, the undermost pair 


