All letters are 'ALS' (autograph letter signed) unless otherwise stated.

The following abbreviations and conventions are used throughout:

• CD – Charles Darwin 
• del – deleted
• illeg – illegible
• interl – interlined
• underl – underlined
• [some text] – 'some text' is an editorial insertion
• [some text] – 'some text' is the conjectured reading of an ambiguous word or passage
• [some text] – 'some text' is a description of a word or passage that cannot be transcribed, eg. '[3 words illeg]'
• 〈    〉 – word(s) destroyed
• 〈some text〉 – 'some text' is a suggested reading for a destroyed word or passage
• 〈some text〉 – 'some text' is a description of a destroyed word or passage, eg. '〈3 lines excised〉'

The following are used in the 'physical description' for each letter.  Letters are assumed to be 'ALS' (letters both written and signed in the sender's own hand) unless otherwise stated. 

• L – letter
• pc – post card
• mem – memorandum
• T – telegram
• D – document (other than above)
• draft – draft of letter
• C – copy of letter
• CC – contemporary copy of letter (e.g., copy kept by sender)
• trans – translation of letter (into or from English)
• A – autograph (½ or more of text in sender’s hand)
• (A) – part autograph (any portion less than ½ in sender’s hand)
• S – signed by sender
• (S) – signed with sender’s name by amanuensis
• ALS – autograph letter signed by sender
• LS – letter in hand of amanuensis, signed by sender
• LS(A) – LS with additions or emendations in sender’s hand
• TLS – typed letter signed
• ACCS – contemporary copy in sender’s hand, signed by him/her
• CCS(A) – contemporary copy signed, with additions by sender in his/her own hand
• (French) – language of described item
• inc – incomplete (part(s) of the original document missing)
• damaged – part(s) of original document destroyed
• † – annotations by recipient
• †(by Lyell) – annotations by a specified person
• †† – lengthy, numerous, or particularly important annotations by recipient
• CD note – separate note by Darwin relating directly to described item and usually physically associated with it
• Lyell note – separate note by a specified person relating directly to described item and usually physically associated with it
• encl – enclosure
• cov – cover or envelope

So eg:  

ALS 5pp (French) inc & damaged ††, encl memS 6pp †.

Means:  

a five-page autograph letter signed by the sender and written in French. The letter is incomplete and damaged and has numerous, lengthy, or important annotations by the recipient (Darwin); enclosed are six pages of notes, signed by the sender and annotated by the recipient.