344 Mr. Jeffreys on a Species o/Limopsis 



that the animal of Limopsis is closely allied to that of Peclun- 

 cuius in form ; and the same remark applies to the shell in each 

 of these genera. 



The shell of Limopsis differs from that of Pcctunculus only in 

 the apparatus by which it is closed. In Pcctunculus the hinge- 

 area or plate at the back of the shell is furnished with a strong 

 ligament which extends from one side to the other, and unites 

 the whole of the hinge; while in Limopsis there is no ligament 

 (properly so called), but only a small cartilage which fits into a 

 triangular pit or depression placed just under the beak in each 

 valve. The ligament and cartilage are in either case exterior to 

 and above the hinge-line, but are seated within the beaks. In 

 specimens of Limopsis of every age the teeth are continuous ; 

 but in Pcctunculus they are interrupted in the middle, and form 

 two distinct rows which obliquely diverge from the centre. But 

 this last character I do not regard as constant or of much 

 value. 



In each of these genera, specimens of the same species vary 

 greatly in being more or less oblique, and in the hinge-margin 

 occasionally projecting at each end to such an extent as to give 

 the shell an appearance of having ears, as is the case in many 

 other genera of the family of Arcidae. 



Nucula has also a cartilage with a pit for its reception ; but 

 in that genus these processes are internal, instead of external as 

 in Limopsis, and they are placed helow instead of above the 

 hinge-line. 



IJma bears no affinity to Limopsis, having only a single ad- 

 ductor muscle, and a different kind of hinge-fastening. 



The history of the genus Limopsis is involved in some obscu- 

 rity, which is partly owing to the rarity of the work in which it 

 was originally described. According to the 'Lethsea geognos- 

 tica^ of Bronn (whose loss is so deeply felt and deplored by geo- 

 logists), Limopsis was proposed as a new genus by Sassi, in the 

 ' Giornale Ligustrico ' for 1827 ; but the British Museum library 

 does not contain any such periodical, and I have not been able 

 to obtain a sight of it in this country. Nyst, in his work on 

 Belgian fossils (1843), stated that he was ignorant of Sassi's 

 publication or its date, although Bronn had given both these 

 particulars twelve years before the above statement was made. 

 Nyst and Galeotti had in 1835 proposed for the same genus the 

 name of TrigonoccBlia. The late Prof. d^Orbigny, equally dis- 

 regarding the rule of priority in scientific nomenclature, pub- 

 lished, in the 'Paleontologie Fran9aise^ (1844), another name 

 (Pectunculina), which has been lately adopted by Dr. Chenu, who 

 makes Limopsis and Trigonoccclia subgenera of D'Orbigny's 

 genus, but upon what grounds it is almost impossible to ima- 



